UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, D.C. 20549 FORM 11-K [X] ANNUAL REPORT PURSUANT TO SECTION 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2005 ----------------- or [ ] TRANSITION REPORT PURSUANT TO SECTION 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the transition period from ____________ to____________ Commission File Number: 1-9894 A. Full title of the plan and address of the plan, if different from that of the issuer named below: ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN B. Name of issuer of the securities held pursuant to the plan and the address of its principal executive office: ALLIANT ENERGY CORPORATION 4902 North Biltmore Lane Madison, Wisconsin 53718 REQUIRED INFORMATION The following financial statements and schedules of the Alliant Energy Corporation 401(k) Savings Plan, prepared in accordance with the financial reporting requirements of the Employee Retirement Income Security Act of 1974, as amended, are filed herewith. Page 1 of 21 pages Exhibit Index is on page 20 1 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN FINANCIAL STATEMENTS AS OF DECEMBER 31, 2005 AND 2004 AND FOR THE YEAR ENDED DECEMBER 31, 2005, SUPPLEMENTAL SCHEDULES FOR THE YEAR ENDED DECEMBER 31, 2005, AND REPORT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM 2 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN TABLE OF CONTENTS Page No. -------- REPORT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM 4 FINANCIAL STATEMENTS Statements of Net Assets Available for Benefits as of December 31, 2005 and 2004 5 Statement of Changes in Net Assets Available for Benefits for the Year Ended December 31, 2005 6 NOTES TO FINANCIAL STATEMENTS 7-12 SUPPLEMENTAL SCHEDULES Form 5500, Schedule H, Part IV, line 4i - Schedule of Assets (Held at End of Year) as of December 31, 2005 13-16 Form 5500, Schedule H, Part IV, line 4i - Schedule of Assets (Acquired and Disposed of Within Year) for the Year Ended December 31, 2005 17 Form 5500, Schedule H, Part IV, line 4j - Schedule of Reportable Transactions for the Year Ended December 31, 2005 18 SIGNATURES 19 EXHIBIT INDEX 20 Consent of Independent Registered Public Accounting Firm 21 3 Report of Independent Registered Public Accounting Firm To the Total Compensation Committee and Participants of the Alliant Energy Corporation 401(k) Savings Plan: We have audited the accompanying statements of net assets available for benefits of Alliant Energy Corporation 401(k) Savings Plan (the "Plan") as of December 31, 2005 and 2004, and the related statement of changes in net assets available for benefits for the year ended December 31, 2005. These financial statements are the responsibility of the Plan's management. Our responsibility is to express an opinion on these financial statements based on our audits. We conducted our audits in accordance with standards of the Public Company Accounting Oversight Board (United States). Those standards require that we plan and perform the audit to obtain reasonable assurance about whether the financial statements are free of material misstatement. The Plan is not required to have, nor were we engaged to perform, an audit of its internal control over financial reporting. Our audits included consideration of internal control over financial reporting as a basis for designing audit procedures that are appropriate in the circumstances, but not for the purpose of expressing an opinion on the effectiveness of the Plan's internal control over financial reporting. Accordingly, we express no such opinion. An audit also includes examining, on a test basis, evidence supporting the amounts and disclosures in the financial statements, assessing the accounting principles used and significant estimates made by management, as well as evaluating the overall financial statement presentation. We believe that our audits provide a reasonable basis for our opinion. In our opinion, such financial statements present fairly, in all material respects, the net assets available for benefits of the Plan as of December 31, 2005 and 2004, and the changes in net assets available for benefits for the year ended December 31, 2005 in conformity with accounting principles generally accepted in the United States of America. Our audits were conducted for the purpose of forming an opinion on the basic financial statements taken as a whole. The supplemental schedules of (1) assets (held at end of year) as of December 31, 2005, (2) assets (acquired and disposed of within year) for the year ended December 31, 2005, and (3) reportable transactions for the year ended December 31, 2005, are presented for the purpose of additional analysis and are not a required part of the basic financial statements, but are supplementary information required by the Department of Labor's Rules and Regulations for Reporting and Disclosure under the Employee Retirement Income Security Act of 1974. These schedules are the responsibility of the Plan's management. Such schedules have been subjected to the auditing procedures applied in our audit of the basic 2005 financial statements and, in our opinion, are fairly stated in all material respects when considered in relation to the basic financial statements taken as a whole. /s/ Deloitte & Touche LLP ------------------------- Milwaukee, Wisconsin June 28, 2006 4 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN STATEMENTS OF NET ASSETS AVAILABLE FOR BENEFITS December 31, 2005 2004 --------------------- -------------------- Investment income receivable $1,916,893 $660,002 Contributions receivable 1,256,949 374,368 --------------------- -------------------- Total receivables 3,173,842 1,034,370 --------------------- -------------------- Investments, other than participant promissory notes (Refer to Notes 7 and 8) 487,229,569 453,499,612 Participant promissory notes 8,837,772 8,742,835 --------------------- -------------------- Total investments 496,067,341 462,242,447 --------------------- -------------------- Net assets available for benefits $499,241,183 $463,276,817 ===================== ==================== The accompanying Notes to Financial Statements are an integral part of these statements. 5 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN STATEMENT OF CHANGES IN NET ASSETS AVAILABLE FOR BENEFITS For the Year Ended December 31, 2005 Net assets available for benefits - beginning of year $463,276,817 Contributions: Cash contributions from employees 26,344,221 Employer contributions: Cash for purchase of Alliant Energy Corporation common stock 7,682,202 Cash 347,587 Rollovers from other qualified plans 507,360 Investment income: Net appreciation in fair value of investments (Refer to Note 7) 17,319,924 Interest and dividends 16,027,496 Distributions to participants (32,264,424) ------------------- Net assets available for benefits - end of year $499,241,183 =================== The accompanying Notes to Financial Statements are an integral part of this statement. 6 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN NOTES TO FINANCIAL STATEMENTS FOR THE YEAR ENDED DECEMBER 31, 2005 Note 1. Description of the Plan The Alliant Energy Corporation 401(k) Savings Plan (the Plan) is a qualified defined contribution plan under Section 401(k) of the Internal Revenue Code (the Code), as amended, and meets the applicable requirements of the Employee Retirement Income Security Act of 1974, as amended (ERISA). The following brief description of the Plan is provided for general information purposes only. More complete information regarding the Plan is provided in the plan document and summary plan description, which have been made available to all eligible Plan participants (participants). The Plan is administered by the Total Compensation Committee (the Committee) and the Plan sponsor is Alliant Energy Corporate Services, Inc. (a direct subsidiary of Alliant Energy Corporation). The Committee reserves the right to terminate, amend or modify the Plan if future conditions warrant such action. Any regular employee of Alliant Energy Corporation and its participating subsidiaries (the Company) age 18 and over may participate in the Plan. Regular full-time employees and regular part-time employees customarily scheduled to work at least half-time may participate immediately following 30 days of service. Part-time employees customarily scheduled to work less than half-time may participate after 12 months of service during which he or she has earned at least 1,000 paid hours. An Employee Stock Ownership Plan (ESOP) is in place within the Plan. Under these provisions, participants have the option to elect to receive cash for any dividends paid on Company common stock within the Plan or to have the dividends reinvested in additional shares based on the current market price. Also, the Company is eligible for the dividend deduction provisions of Section 404(k) of the Code for Company common stock held in the ESOP. The Company provides matching contributions for amounts contributed by participants to the Plan. Other than the exceptions noted below, Company match information and employee contribution limits for 2005 were as follows: Company match for each $1 contributed $0.50 Maximum amount of Company match as a percentage of each respective participant's eligible compensation 3% Eligible employee annual contribution limit as a percentage of compensation 50% Maximum annual contribution limit* $14,000 *Participants who were at least age 50 by Dec. 31, 2005 were eligible to make additional catch-up contributions of up to $4,000 in 2005. These additional catch-up contributions were not eligible for any Company match. Company contributions for Alliant Energy Resources, Inc. (Resources) and Cedar Rapids and Iowa City Railway Company (CRANDIC) employees are calculated based on a percentage of base pay, without overtime or incentive pay, and there is a "basic" Company contribution equal to 4% and 2%, respectively, of base pay. Resources and CRANDIC employees may be eligible for a "discretionary" Company contribution of $0.50 for every $1 contributed up to a maximum of the first 6% of pay in addition to the "basic" Company contribution. The "discretionary" contribution for both Resources and CRANDIC employees is based on achieving specified goals and is typically determined and paid during the first quarter of the following year. The "discretionary" Company contributions for 2005 and 2004 were $198,808 and $374,368, respectively. An "additional" Company contribution is allocated to the accounts of active participants, except for Resources and CRANDIC employees, as of the last day of the Plan year, who contributed at least 6% of compensation during the Plan year and did not receive a Company matching contribution equal to 3% of compensation. The amount of the "additional" Company contribution is the difference between 3% of compensation during the Plan year and the amount of Company matching contributions previously received during the Plan year. 7 Beginning Jan. 1, 2006, the Company began making changes to the Plan for newly hired non-bargaining employees and all Resources and CRANDIC employees. For all other non-bargaining employees, the following changes will go into effect on Aug. 1, 2008. The changes include the Company matching $0.50 for each $1 contributed up to the first 8% of each respective participant's eligible compensation. In addition, the Company provides a "cash contribution" into each active employees' 401(k) account each pay period based on a percentage of base pay as follows: Company Cash Age plus Years of Service Contribution ------------------------- -------------- 49 or less 4% 50 - 69 5% 70+ 6% Company contributions are invested in the Alliant Energy Corporation Common Stock Fund, except for Company "basic" contributions for Resources and CRANDIC employees and beginning Jan. 1, 2006, the "cash contribution" for newly hired non-bargaining employees and all Resources and CRANDIC employees. These exceptions are invested at each participant's discretion. Participants are immediately vested in their respective employee and employer contributions. Contributions under the Plan are held and invested, until distribution, in a Trust Fund maintained by Ameriprise Trust Company (the Trustee), formerly known as (f/k/a) American Express Trust Company, a division of Ameriprise Financial Services, Inc. Participants may subsequently redesignate the distribution of future contributions or transfer existing balances between investment funds on a daily basis, subject to the limits set forth in the Plan. As previously described, Company matching contributions invested in the Alliant Energy Corporation Company Stock Fund may not be transferred to any other investment fund. However, Company matching contributions may be transferred by certain participants during the 30-day period immediately prior to retirement and participants age 55 with 10 years of service are eligible to diversify up to 100% of their ESOP account to one or more of the investment options. The Plan has provisions under which participants who are active employees may take loans up to the lesser of $50,000 or 50% of their total account balance (a $1,000 minimum loan amount and a maximum of three loans for each participant also apply). The Committee determines the loan interest rate pursuant to the Plan. Interest rates on participant loans ranged from 5.0% to 10.5% at both Dec. 31, 2005 and 2004. Principal and interest are repaid bi-weekly through employee payroll deductions. Note 2. Summary of Significant Accounting Policies (a) Basis of Accounting - The financial statements have been prepared in accordance with accounting principles generally accepted in the United States of America (GAAP). (b) Valuation of Investments and Income Recognition - All Guaranteed Investment Contracts (GIC's) held by the Plan are fully benefit responsive and are valued at contract value, which represents contributions made under the contract, plus interest at the contract rate (less funds used to pay Plan benefits). The carrying value of all GIC investments was $38,204,962 and $35,339,962 at Dec. 31, 2005 and 2004, respectively. The approximate fair value of these investments was $46,887,620 and $43,639,028 at Dec. 31, 2005 and 2004, respectively, based on the discounted cash flows valuation method. Under the terms of the GIC's, the weighted average crediting interest rate was 4.33% and 5.00% on Dec. 31, 2005 and 2004, respectively, and is reset quarterly for all contracts. The GIC's earned an average rate of 4.86% and 4.80% in 2005 and 2004, respectively. Participant loans are carried at unpaid principal balances due. All other Plan investments are carried at fair value as determined by quoted market prices, except for the RiverSource Trust Income Fund I, f/k/a American Express Trust Income Fund I. The RiverSource Trust Income Fund I is a common collective trust fund, which includes investments in fully benefit responsive GIC's and is reported at contract value, which approximates fair value. Interest income is accrued when earned. Dividend income is recorded on the ex-dividend date. Investment transactions are recorded on the trade date. 8 (c) Net Appreciation in Fair Value of Investments - Net realized and unrealized appreciation is recorded in the accompanying statement of changes in net assets available for benefits as "Net appreciation in fair value of investments." (d) Payment of Benefits - Benefit payments to participants are recorded when paid. (e) Expenses - Investment management fees are paid from investment earnings prior to crediting earnings to the individual participant account balances. Most other Plan administrative expenses are absorbed by the Company. Expenses incurred in maintaining Self-Managed Brokerage Accounts are the responsibility of the respective Plan participants. (f) Use of Estimates - The preparation of financial statements in conformity with GAAP requires the Plan administrator to make estimates and assumptions that affect the reported amounts of net assets available for benefits at the date of the financial statements and the reported amounts of changes in net assets available for benefits during the reporting period. Actual results could differ from those estimates. (g) Risk and Uncertainties - The Plan invests in various investments, including common investment funds, mutual funds, common stock of the Company and investment contracts. The Plan also offers a Self-Managed Brokerage Account option which allows participants to invest in a wide range of mutual funds. Investments, in general, are exposed to various risks, such as interest rate, credit, and overall market volatility. Due to the level of risk associated with certain investments, it is reasonably possible that changes in the values of certain investments will occur in the near term and that such changes could materially affect the amounts reported in the financial statements. (h) Reclassifications - Certain prior period amounts have been reclassified on a basis consistent with the current year presentation. Note 3. Tax Status The Internal Revenue Service has determined and informed the Company by a letter dated Aug. 25, 2003, that the Plan and related trust are designed in accordance with the applicable sections of the Code. The Plan has been amended since receiving the determination letter. However, the Committee and the Plan's tax counsel believe that the Plan is designed and is currently being operated in compliance with the applicable requirement of the Code. Note 4. Plan Termination Provisions Upon termination of the Plan in its entirety, each participant is entitled to receive, in accordance with the terms of the Plan, the entire balance in their account. The Company has no intention to terminate the Plan. 9 Note 5. Withdrawals and Distributions Withdrawals from participants' account balances are allowed when participants who are actively employed reach age 70-1/2 (or age 59-1/2 for certain participants). Withdrawals are also allowed due to special "hardship" circumstances. Distributions from the Plan will be made upon termination of employment (by retirement, death, disability or otherwise) if the participant's account balance is less than $5,000. If a withdrawing participant's account balance exceeds $1,000 but is less than $5,000, and the participant does not make an election to either have the account paid as a direct rollover or as a cash payment, the distribution will be paid as a direct rollover to an individual retirement account established at the Trustee. If a withdrawing participant's account balance exceeds $5,000, the participant may elect to defer payment until he or she is age 70-1/2. Distributions can be either in the form of a lump sum or substantially equal annual installments. The unpaid portion of all loans made to the participant, including accrued interest, will be deducted from the amount of the participant account to be distributed. Distributions payable to participants at Dec. 31, 2005 and 2004 were $59,982 and $25,656, respectively. Distributions payable are not included within net assets available for benefits in the accompanying financial statements, however, they are recorded as liabilities in the Plan's Form 5500 (refer to Note 10). Note 6. Derivative Financial Instruments The Plan did not invest in any material derivative financial instruments during 2005 and 2004. Note 7. Other Investment Information Investments held which were greater than 5% of the Plan's net assets available for benefits as of Dec. 31 were as follows: 2005 2004 ----------------- ------------------ Alliant Energy Corporation Common Stock* $107,427,885 $108,063,758 (non-participant directed: $68,338,101 (2,437,165 shares) and $63,508,962 (2,220,593 shares), respectively) (participant directed: $39,089,784 (1,394,072 shares) and $44,554,796 (1,557,860 shares), respectively) American Funds Growth Fund of America, 2,660,335 and 2,572,065 shares, respectively 82,097,953 70,423,142 RiverSource Trust Equity Index Fund III*, f/k/a American Express Trust Equity Index Fund III, 2,209,051 and 2,340,750 shares, respectively 76,095,171 76,942,777 Dodge & Cox Stock Fund, 300,943 and 196,080 shares, respectively 41,295,339 25,533,595 American Funds EuroPacific Growth Fund, 741,088 and 526,727 shares, respectively 30,110,419 18,572,401 PIMCO Total Return Fund, 2,780,718 and 1,941,165 shares, respectively 29,197,536 20,712,227 Brown Capital Management Small Company Fund, 24,349 and 822,599 shares, respectively 760,668 24,513,456 RiverSource Trust Long-Term Horizon (65:35) Fund*, f/k/a American Express Trust Long-Term Horizon (65:35) Fund, 0 and 2,660,902 shares, respectively -- 33,931,822 * Represents party known to be a party-in-interest to the Plan. 10 During 2005, the Plan's investments, including gains and losses on investments acquired and disposed of, as well as held during the year, appreciated (depreciated) in value as follows: American Funds Growth Fund of America $9,322,356 American Funds EuroPacific Growth Fund 3,749,899 RiverSource Trust Equity Index Fund III*, f/k/a American Express Trust Equity Index Fund III 3,644,444 Dodge & Cox Stock Fund 2,120,745 State Street Global Advisors S&P Midcap Index Fund 1,314,412 Brown Capital Management Small Company Fund 908,832 Dreyfus Premier Emerging Markets Fund 799,408 RiverSource Trust Income Fund I*, f/k/a American Express Trust Income Fund I 406,206 Evergreen Small Cap Value Fund 288,397 RiverSource Trust Bond Index Fund II*, f/k/a American Express Trust Bond Index Fund II 181,115 Self-Managed Brokerage Accounts 8,768 RiverSource Trust Short-Term Horizon (25:75) Fund*, f/k/a American Express Trust Short-Term Horizon (25:75) Fund (3,470) RiverSource Trust Long-Term Horizon (80:20) Fund*, f/k/a American Express Trust Long-Term Horizon (80:20) Fund (32,912) PIMCO Total Return Fund (428,885) RiverSource Trust Long-Term Horizon (65:35) Fund*, f/k/a American Express Trust Long-Term Horizon (65:35) Fund (637,563) Alliant Energy Corporation Common Stock* (2,022,776) Buffalo Small Cap Fund (2,299,052) ---------------------- Net appreciation in fair value of investments $17,319,924 ====================== * Represents party known to be a party-in-interest to the Plan. Note 8. Non-Participant Directed Investments Information about the net assets and the significant components of the changes in net assets relating to the non-participant directed investments was as follows: Dec. 31, Net assets: 2005 2004 ---------------------- ---------------------- Alliant Energy Corporation Common Stock* $68,338,101 $63,508,962 RiverSource Trust Money Market Fund I*, f/k/a American Express Trust Money Market Fund I* 987,313 993,002 Contributions receivable 378,296 374,368 Investment income receivable 5,358 697 ---------------------- ---------------------- $69,709,068 $64,877,029 ====================== ====================== Changes in net assets: 2005 ---------------------- Employer contributions $7,682,202 Investment activity: Investment income 2,653,378 Net depreciation in fair value of investments (1,331,372) Transfers to participant directed investments (1,048,064) Distributions to participants (3,124,105) ---------------------- $4,832,039 ====================== * Represents party known to be a party-in-interest to the Plan. 11 Note 9. Related Party Transactions Certain Plan investments are shares of mutual funds and common trust funds managed by an affiliate of the Trustee and shares of common stock of the Company. As of Dec. 31, 2005 and 2004, the Plan held 3,831,237 and 3,778,453 shares of Alliant Energy Corporation common stock with a cost basis of $94,376,693 and $92,640,650, respectively. In 2005 and 2004, the Plan recorded dividend income of $3,980,853 and $3,705,802, respectively, from investments in common stock of the Company. These transactions qualify as party-in-interest. Note 10. Reconciliation to Form 5500 Distributions payable to participants are not included within net assets available for benefits in the accompanying financial statements, however, they are recorded as liabilities in the Plan's Form 5500. The following table reconciles net assets available for benefits per the financial statements to the Plan's Form 5500 as filed by the Company: 2005 2004 ---------------- ----------------- Net assets available for benefits per financial statements $499,241,183 $463,276,817 Adjustments: Benefits requested not yet paid (59,982) (25,656) ---------------- ----------------- Amounts reported per Form 5500 $499,181,201 $463,251,161 ================ ================= The following table reconciles distributions to participants per the financial statements to the Form 5500 as filed by the Company for 2005: Distributions to Participants ---------------- Amounts reported per financial statements ($32,264,424) Adjustments: Changes in benefits requested not yet paid (34,326) ---------------- Amounts reported per Form 5500 ($32,298,750) ================ 12 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN FORM 5500, SCHEDULE H, PART IV, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) AS OF DECEMBER 31, 2005 Identity of issue, borrower, Description of investment including maturity date, Current lessor, or similar party rate of interest, collateral, par or maturity value Cost Value -------------------------------- --------------------------------------------------------------- ---------------- ----------- Registered Investment Companies Evergreen Small Cap Value Fund, 397,516 shares $9,134,595 $9,607,957 American Funds Growth Fund of America, 2,660,335 shares 55,490,909 82,097,953 PIMCO Total Return Fund, 2,780,718 shares 29,813,794 29,197,536 Dodge & Cox Stock Fund, 300,943 shares 35,313,353 41,295,339 Brown Capital Management Small Company Fund, 24,349 shares 663,091 760,668 Dreyfus Premier Emerging Markets Fund, 604,841 shares 12,104,979 13,076,672 American Funds EuroPacific Growth Fund, 741,088 shares 24,421,136 30,110,419 Buffalo Small Cap Fund, 843,993 shares 23,739,855 21,471,186 Common/Collective Trusts RiverSource Trust Money Market Fund I*, 6,864,577 shares 6,864,577 6,864,577 RiverSource Trust Equity Index Fund III*, 2,209,051 shares 65,579,179 76,095,171 RiverSource Trust Income Fund I*, 97,652 shares 6,520,606 6,720,671 RiverSource Trust Investment Grade Bond Fund*, 562,422 shares 8,870,771 9,372,206 RiverSource Trust Bond Index Fund II*, 544,433 shares 7,751,573 7,910,067 State Street Global Advisors S&P Midcap Index Fund, 339,474 shares 9,906,546 11,541,098 Corporate Stocks: Common Alliant Energy Corporation common stock*, 3,831,237 shares 94,376,693 107,427,885 Corporate Bonds ARMT 2005 - 3 - 7A1, 5.09%, 4/25/35, par value $135,355 136,267 134,292 AMCAR 2003 - CF A3, 2.75%, 10/9/07, par value $45,164 45,868 45,079 AMCAR 2004 - AF A3, 2.18%, 7/7/08, par value $127,270 127,262 126,355 AMCAR 2005 - BM - A3, 4.05%, 1/6/10, par value $300,000 299,978 296,672 AMERICREDIT AUTOMOBILE REC., 4.87%, 12/6/10, par value $200,000 199,974 200,031 AMSI 2005 - R9 - AF3, 5.10%, 11/25/35, par value $315,000 315,000 313,622 BACM 2005 - 4 - A1, 4.43%, 7/10/45, par value $136,248 136,251 134,651 BACM 2005 - 6 - A2, 5.19%, 9/10/47, par value $175,000 175,882 175,848 BOAA 2003 - 1 - A1, 5.00%, 2/25/33, par value $101,613 102,295 100,804 BOAMS 2004 - E 2A6, 4.13%, 6/25/34, par value $150,000 148,781 144,839 BALTA 2005 - 5 - 1 A1, 4.80%, 6/25/35, par value $169,247 169,247 169,459 BSCMS 2005 - PWR9 - A1, 4.50%, 9/15/42, par value $213,647 214,181 211,058 BSCMS 2005 - T20 - A2, 5.13%, 10/12/42, par value $250,000 251,366 251,268 CDC COMMERCIAL MTGE, 5.68%, 11/15/30, par value $175,000 178,192 179,593 CDBCM 2005 - CD1 A1, 5.05%, 9/15/20, par value $247,950 248,569 248,324 CWALT 2005 - 6CB - 1A1, 7.50%, 4/25/35, par value $60,810 64,867 63,139 CWL 2005 - 10 - AF6, 4.91%, 12/25/35, par value $20,000 19,999 19,388 CWL 2005 - 17 - 1AF2, 5.36%, 12/25/35, par value $125,000 124,998 125,072 CWALT INC 2005 - 54CB, 5.50%, 11/25/35, par value $67,972 69,077 68,612 CWALT INC 2005 - 43, 5.50%, 11/25/35, par value $72,681 73,726 73,227 CARAT 2005 - 1 - A4, 4.05%, 7/15/09, par value $250,000 249,987 245,548 COAFT 2002 - B A4A, 3.32%, 4/15/09, par value $73,263 74,706 72,765 COAFT 2003 - A A3A, 1.83%, 10/15/07, par value $14,257 14,207 14,229 COAFT 2004 - A A3, 3.07%, 7/15/08, par value $100,000 99,999 99,084 CGCMT 2005 - C3 - A1, 4.39%, 5/15/43, par value $282,394 283,095 278,275 CMLTI 2005 - 5 - 1A2, 5.41%, 8/25/35, par value $296,031 297,153 295,846 CSFB 2004 - C1 A2, 3.52%, 1/15/37, par value $100,000 100,500 96,402 CSFB 2005 - C4 - A1, 4.77%, 8/15/38, par value $214,519 215,056 213,399 CSFB 2005 - 8 - 7A1, 7.00%, 9/25/35, par value $159,081 164,600 163,518 CSFB 2005 - 10 - 4A1, 6.50%, 11/25/35, par value $98,241 100,260 99,899 CSFB - 05 - 12 - SA1, 7.00%, 12/25/35, par value $150,000 155,508 155,203 FRNK 2004 - 2 - A4, 3.93%, 8/15/12, par value $175,000 173,496 171,252 GMACC 2005 - C1 - A1, 4.21%, 5/10/43, par value $94,416 94,650 92,895 GMACC 2002 - C2 - A1, 4.32%, 10/15/38, par value $115,907 117,972 115,357 13 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN FORM 5500, SCHEDULE H, PART IV, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) AS OF DECEMBER 31, 2005 (Continued) Identity of issue, borrower, Description of investment including maturity date, Current lessor, or similar party rate of interest, collateral, par or maturity value Cost Value -------------------------------- --------------------------------------------------------------- ------------- ----------- Corporate Bonds (continued) GMACM 2004 - HE2 - A4, 3.65%, 10/25/33, par value $95,000 91,875 91,568 GSR 2005 - AR2 - 2A1, 4.88%, 4/25/35, par value $237,103 238,363 234,429 GECMC 2005 - C3 - A2, 4.85%, 7/10/45, par value $350,000 351,746 347,461 GCCFC 2005 - GG5 - A1, 4.79%, 4/10/37, par value $247,333 247,332 246,861 HVMLT 05 - 4 3A, 5.178%, 1/19/35, par value $204,021 205,822 203,655 HVMLT 2005 - 12 - 2A1A, 5.62%, 10/19/35, par value $146,728 150,029 149,927 HERTZ VEHICLE FINANCING, 4.93%, 2/15/10, par value $150,000 149,991 149,813 HART 2003 - A A3, 2.33%, 11/15/07, par value $78,436 78,435 77,896 INDX 2005 - AR25 - A1, 5.91%, 12/25/35, par value $99,204 100,216 99,953 JPMCC 2003 - C1 - A1, 4.27%, 1/12/37, par value $197,987 195,953 192,958 JP MORGAN CHASE COMM MTGE, 4.65%, 8/15/42, par value $262,280 262,930 260,410 JPMCC 2005 - LDP4 - A1, 4.61%, 10/15/42, par value $269,958 270,820 267,616 LBUBS 2004 - C4 A2, 4.57%, 5/15/34, par value $150,000 153,697 148,310 LBUBS 2004 - C6 A2, 4.19%, 8/15/29, par value $200,000 200,996 195,406 LBUBS 2003 - C7 A2, 4.06%, 9/15/27, par value $185,000 186,356 179,709 LBUBS 2003 - C8 A2, 4.21%, 11/15/27, par value $150,000 150,744 146,994 LBUBS 2005 - C1 - A1, 4.06%, 2/15/30, par value $88,493 88,934 87,025 LBUBS 2005 - C3 - A1, 4.39%, 7/15/30, par value $201,086 202,091 199,005 LBUBS 2005 - C5 - A2, 4.88%, 9/15/30, par value $280,000 281,398 278,720 LBART 2005 - B - A3, 4.41%, 5/15/10, par value $200,000 200,000 198,187 MARM 2004 - 3 3A2, 4.42%, 4/25/34, par value $80,115 81,718 79,087 MLMI 2005 - A1 - 2A1, 4.63%, 1/25/35, par value $82,308 82,632 81,019 MLMT 2005 - CIP1 - A1, 4.63%, 5/12/10, par value $210,447 210,446 208,666 MLMT 2005 - CK1 - A1, 5.03%, 11/12/37, par value $175,000 175,434 175,901 MSC 2003 - T11 A2, 4.34%, 6/13/41, par value $250,000 243,213 244,232 MSM 2004 - 2AR 3A, 5.03%, 2/25/34, par value $142,431 145,858 140,859 POPLR 2005 - 5 - AF3, 5.09%, 11/25/35, par value $175,000 174,999 174,180 RAMC 2005 - 3 - AF3, 4.77%, 10/25/35, par value $100,000 99,998 98,938 RAMC 2005 - 4 - A3, 5.56%, 2/25/36, par value $70,000 70,000 70,000 RALI SER 2003 - QS4 ABS, 5.50%, 9/25/33, par value $68,751 70,679 68,785 RASC 2004 - KS8 AI3, 3.84%, 9/25/34, par value $125,000 124,980 123,106 RASC SER 2002 - KS1, 5.86%, 11/25/29, par value $493 525 491 SARM 2005 - 15 - 4A1, 5.53%, 7/25/35, par value $208,161 210,072 207,771 SASCO CMO 2004 - 12H, 5.53%, 4/25/34, par value $65,661 67,141 66,002 TRIAD AUTO RECEIVABLES 2005 - A, 4.05%, 3/12/10, par value $250,000 249,997 247,113 WESTO 2003 - 4 A3, 2.39%, 1/22/08, par value $42,997 42,995 42,909 WBCMT 2003 - C8 - A2, 3.89%, 11/15/35, par value $150,000 150,745 145,406 WBCMT 2005 - C19 - A1, 4.17%, 6/15/42, par value $166,464 166,460 163,399 WBCMT 2005 - C16 - A2, 4.38%, 10/15/41, par value $100,000 100,498 97,601 WBCMT 2005 - C21 - A1, 4.87%, 10/17/44, par value $220,488 221,588 220,750 WAMU 2005 - AR3 - A2, 4.65%, 3/25/35, par value $130,469 130,576 128,214 WFMBS 2005 - 5 - 3PT3, 5.50%, 5/25/35, par value $296,596 295,907 291,962 ABSA 2005 - CF A4, 4.63%, 6/6/12, par value $300,000 299,998 297,543 CS FIRST BOSTON MTGE SECURITIES, 5.10%, 8/15/38, par value $250,000 249,793 250,302 VENDEE 2003 - 2 C, 5.00%, 7/15/20, par value $76,992 79,615 76,881 WFMBS, 5.00%, 10/25/35, par value $337,553 327,252 326,055 14 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN FORM 5500, SCHEDULE H, PART IV, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) AS OF DECEMBER 31, 2005 (Continued) Identity of issue, borrower, Description of investment including maturity date, Current lessor, or similar party rate of interest, collateral, par or maturity value Cost Value --------------------------------- ----------------------------------------------------------------- ---------- ----------- Government and Agency Obligations FHLMC GOLD #E91326, 6.50%, 9/1/17, par value $67,063 71,045 68,902 FHLMC GOLD #E96180, 4.50%, 5/1/13, par value $108,716 112,793 106,743 FHLMC GOLD #E96579, 4.50%, 6/1/13, par value $129,693 134,495 126,561 FHLMC GOLD #E97247, 5.00%, 6/1/18, par value $104,332 106,549 103,509 FHLMC MTN, 3.525%, 12/20/07, par value $370,000 370,000 361,651 FHLMC GOLD #C66932, 6.00%, 5/1/32, par value $56,626 56,750 57,290 FHLMC GOLD #B10776, 5.00%, 11/1/13, par value $97,493 100,082 97,133 FHLB FLT RT NT, 4.73%, 9/6/07, par value $330,000 330,000 330,000 FED HOME LN BANK, 4.625%, 1/18/08, par value $540,000 539,352 539,228 FHLB, 3.00%, 5/15/06, par value $215,000 215,841 213,745 FED FARM CREDIT BANK, 2.38%, 10/2/06, par value $150,000 150,473 147,472 FEDERAL FARM CREDIT BANK, 2.13%, 7/17/06, par value $250,000 249,523 246,835 FHLMC REF NOTE, 5.13%, 10/15/08, par value $125,000 128,745 126,289 FHLMC, 4.63%, 12/19/08, par value $575,000 573,195 574,005 FHLMC (NON GOLD) ARM #782436, 4.99%, 10/1/34, par value $97,708 99,906 96,654 FNMA, 3.88%, 11/17/08, par value $350,000 349,202 342,517 FNMA BENCHMARK, 2.25%, 2/28/06, par value $145,000 143,913 144,471 FNMA, 3.00%, 3/2/07, par value $220,000 219,593 215,614 FNMA BENCHMARK, 4.50%, 10/15/08, par value $85,000 84,867 84,529 FNMA 15YR #252260, 6.00%, 3/1/10, par value $368,089 366,306 376,126 FNMA #254187, 5.00%, 12/1/08, par value $203,392 202,280 202,513 FNMA #254291, 7.00%, 4/1/17, par value $87,937 93,790 91,370 FNMA #254757, 5.00%, 3/31/13, par value $101,758 106,337 100,711 FNMA #254793, 5.00%, 7/1/33, par value $138,290 136,259 134,476 FNMA #357324, 5.00%,1/1/33, par value $334,721 322,274 325,656 FNMA #535170, 5.50%, 9/1/14, par value $103,627 106,930 104,472 FNMA #545701, 7.00%, 7/1/12, par value $88,936 95,050 90,841 FNMA #545864, 5.50%, 8/1/17, par value $171,333 173,863 172,745 FNMA #555740, 4.50%, 9/1/18, par value $215,590 211,211 210,360 FNMA #640996, 7.50%, 5/1/32, par value $61,005 65,275 63,903 FNMA #646147, 7.00%, 6/1/32, par value $177,881 186,386 186,853 FNMA #648349, 6.00%, 6/1/17, par value $69,433 72,623 71,150 FNMA #677377, 5.50%, 1/1/33, par value $358,656 355,069 356,130 FNMA 2002 - W4 A3, 5.30%, 5/25/42, par value $4,396 4,407 4,383 FHLMC 2492 - B, 5.50%, 5/15/13, par value $300,113 313,289 301,280 FNMA #200394, 5.50%, 7/25/23, par value $264,441 276,051 265,362 FHLMC 2617 HD, 7.00%, 6/15/16, par value $95,017 103,212 98,524 FNMA 2003 - 133 GB, 8.00%, 12/25/26, par value $51,426 56,351 54,620 FHLMC 2641, 6.50%, 1/15/18, par value $98,082 104,664 102,621 FNMA 2004 - W3 A14, 3.50%, 5/25/34, par value $32,312 32,413 32,186 FNMA 2004 - W3 A15, 5.00%, 5/25/34, par value $100,000 104,641 99,567 FNMA 2004 - 60 PA, 5.50%, 4/25/34, par value $135,188 139,655 135,694 FHLMC 2657 NT, 5.00%, 1/15/16, par value $136,430 141,993 136,548 FHLMC 2672 NT, 5.00%, 2/15/16, par value $185,045 192,714 185,052 FHLMC 2662 DB, 5.00%, 2/15/16, par value $139,589 145,216 139,550 FHLMC 2750 DB, 4.50%, 5/15/15, par value $150,000 153,586 148,824 FHLMC 2843 - BA, 5.00%, 1/15/18, par value $111,024 113,769 110,657 FHLMC 2907 - AG, 4.50%, 3/15/19, par value $105,334 105,301 103,378 15 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN FORM 5500, SCHEDULE H, PART IV, LINE 4i - SCHEDULE OF ASSETS (HELD AT END OF YEAR) AS OF DECEMBER 31, 2005 (Continued) Identity of issue, borrower, Description of investment including maturity date, Current lessor, or similar party rate of interest, collateral, par or maturity value Cost Value --------------------------------- ----------------------------------------------------------------- ----------- ---------- Government and Agency Obligations FNMA #685433, 6.50%, 3/1/33, par value $108,581 114,663 111,626 (continued) FNMA #705304, 4.91%, 6/1/33, par value $151,594 153,809 149,318 FNMA #725090, 4.81%, 11/1/33, par value $127,717 127,038 123,406 FNMA #725425, 5.50%, 4/1/34, par value $311,561 306,827 309,276 FNMA ARM #725737, 4.53%, 8/1/34, par value $92,946 91,827 92,372 FNMA #725815, 6.00%, 12/1/33, par value $205,805 207,638 207,954 FNMA #735057, 4.50%, 1/1/19, par value $184,727 177,627 180,167 FNMA #735935, 5.00%, 12/1/18, par value $347,119 341,315 343,959 FNMA #741897, 5.00%, 10/1/33, par value $105,553 101,925 102,642 FNMA #763798, 5.50%, 3/1/34, par value $281,339 284,856 279,136 FNMA #764082, 4.76%, 1/1/34, par value $103,655 105,356 103,467 FNMA ARM #799769, 5.06%, 11/1/34, par value $119,384 121,772 118,632 FNMA ARM #801344, 5.07%, 10/1/34, par value $117,854 119,990 116,606 FNMA #809534, 5.07%, 2/1/35, par value $109,608 111,077 109,259 FNMA ARM #813570, 4.40%, 10/1/34, par value $82,456 82,624 81,444 FNMA ARM #826908, 5.15%, 8/1/35, par value $193,568 193,084 191,288 FNMA #844705, 5.86%,12/1/35, par value $172,595 174,267 173,872 U.S. TREASURY BOND, 6.25%, 8/15/23, par value $150,000 182,446 179,121 U.S. TREASURY BOND, 6.00%, 2/15/26, par value $590,000 689,516 695,739 U.S. TREASURY NOTE, 2.25%, 2/15/07, par value $620,000 609,934 605,202 U.S. T-NOTE, 4.13%, 5/15/15, par value $270,000 259,242 264,083 U.S. T-NOTE, 3.625%, 6/30/07, par value $940,000 931,451 929,278 U.S. TREASURY NOTE, 4.25%, 11/30/07, par value $875,000 873,045 872,436 FNMA, 4.25%, 9/15/07, par value $315,000 313,095 312,842 FNMA, 3.875%, 5/15/07, par value $425,000 420,668 420,393 FHLMC, 3.63%, 9/15/06, par value $135,000 134,678 134,028 FEDERAL HOME LOAN BANK, 4.63%, 11/21/08, par value $990,000 987,921 987,596 FNMA 2003 - W18 1A5, 4.61%, 8/25/43, par value $100,000 101,172 99,477 FNMA 2004 - W10 A23, 5.00%, 8/25/34, par value $125,000 128,770 125,184 Investment Contracts Monumental Life Insurance Company, 4.66%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 36,810 Bank of America NA, 4.93%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 2,117 Rabobank International, 4.52%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 148,187 IXIS Capital Markets, 3.84%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 164,697 JP Morgan Chase & Co., 4.23%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 38,531 UBS Warburg, 3.98%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 74,053 State Street Bank and Trust Company, 3.74%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 115,190 AIG Financial Products Corp., 4.01%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 21,772 JP Morgan Chase II, 4.59%, 12/31/50 Synthetic Guaranteed Investment Contract Wrapper - 45,413 Hartford, 5.21%, 11/15/06 2,012,562 2,012,562 Protective Life Insurance Company, 6.12%, 6/15/06 1,063,387 1,063,387 Participant-Directed Brokerage Accounts Self-Managed Brokerage Accounts 155,068 170,867 Participant Promissory Notes* Maximum allowable loan -- $50,000 Various interest rates -- 5.0% to 10.5% Primarily maturing within 5 years - 8,837,772 ------------- ----------- $423,871,360 $496,067,341 ============= =========== * Represents party known to be a party-in-interest to the Alliant Energy Corporation 401(k) Savings Plan. 16 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN FORM 5500, SCHEDULE H, PART IV, LINE 4i - SCHEDULE OF ASSETS (ACQUIRED AND DISPOSED OF WITHIN YEAR) FOR THE YEAR ENDED DECEMBER 31, 2005 Description of investment Identity including maturity date, of issue, borrower, lessor, rate of interest, collateral, Cost of Proceeds of or similar party par or maturity value Acquisitions Dispositions ---------------------------------- ------------------------------------------------------- -------------- -------------- Corporate Bonds ARMT 2005 - 3 - 7A1, 5.09%, 4/25/35 $14,744 $14,645 BACM 2005 - 4 - A1, 4.43%, 7/10/45 3,752 3,752 BALTA 2005 - 5 - 1 A1, 4.80%, 6/25/35 55,753 55,753 BSCMS 2005 - PWR9 - A1, 4.50%, 9/15/42 6,369 6,353 CDBCM 2005 - CD1 A1, 5.05%, 9/15/20 2,055 2,050 CWALT 2005 - 6CB - 1A1, 7.50%, 4/25/35 8,851 8,297 CWALT INC 2005 - 54CB, 5.50%, 11/25/35 2,061 2,028 CWALT INC 2005 - 43, 5.50%, 11/25/35 2,352 2,319 CGMCT 2005 - C3 - A1, 4.40%, 5/15/43 17,650 17,606 CMLTI 2005 - 5 - 1A2, 5.41%, 8/25/35 11,131 11,089 CSFB 2005 - C4 - A1, 4.77%, 8/15/38 10,507 10,481 CSFB 2005 - 8 - 7A1, 7.00%, 9/25/35 9,386 9,071 CSFB 2005 - 10 - 4A1, 6.50%, 11/25/35 1,795 1,759 DCAT 2005 - A A3, 3.49%, 12/8/08 99,990 99,754 GMACC 2005 - C1 - A1, 4.21%, 5/10/43 5,598 5,584 GSR 2005 - AR2 - 2A1, 4.86%, 4/25/35 35,539 35,351 GPMF 2005 - AR3 - 1A1, 4.43%, 6/25/45 100,000 99,958 GCCFC 2005 - GG5 - A1, 4.79%, 4/10/37 2,667 2,667 HVMLT 2005 - 3 - 2A1A, 4.61%, 6/19/35 125,000 124,943 HVMLT 05 - 4 3A, 5.17%, 1/19/35 21,165 20,979 HVMLT 2005 - 12 - 2A1A, 5.62%, 10/19/35 3,346 3,272 HAROT 2005 - 1 - A3, 3.53%, 10/21/08 99,989 98,703 INDX 2005 - AR25 - A1, 5.91%, 12/25/35 804 796 JPMCC 2003 - C1 - A1, 4.28%, 1/12/37 4,289 4,333 JP MORGAN CHASE COMM, 4.66%, 8/15/42 12,752 12,720 JPMCC 2005 - LDP4 - A1, 4.61%, 10/15/42 10,074 10,042 LBUBS 2005 - C1 - A1, 4.06%, 2/15/30 11,564 11,507 LBUBS 2005 - C3 - A1, 4.39%, 7/15/30 19,009 18,914 MLMI 2005 - A1 - 2A1, 4.59%, 1/25/35 17,762 17,692 MLMT 2005 - CIP1 - A1, 4.63%, 5/12/10 9,553 9,553 NAROT 2005 - A - A3, 3.54%, 10/15/08 99,998 98,625 RASC 2004 - KS9 - AII2, 4.48%, 10/25/34 30,030 30,038 SARM 2005 - 15 - 4A1, 5.53%, 7/25/35 12,168 12,058 VWALT 2004 - A - A3, 2.84%, 7/20/07 99,035 99,035 WAMU 2005 - AR2 - 2A2A, 4.75%, 1/25/45 100,000 100,013 WBCMT 2005 - C19 - A1, 4.17%, 6/15/42 13,536 13,536 WBCMT 2005 - C21 - A1, 4.87%, 10/17/44 4,535 4,512 WAMU 2005 - AR3 - A2, 4.65%, 3/25/35 15,470 15,458 WFMBS 2005 - 5 - 3PT3, 5.50%, 5/25/35 6,035 6,039 WFMBS, 5.00%, 10/25/35 1,528 1,565 17 ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN FORM 5500, SCHEDULE H, PART IV LINE 4j - SCHEDULE OF REPORTABLE TRANSACTIONS FOR THE YEAR ENDED DECEMBER 31, 2005 Identity Total Total Total Net Adjusted Net of Party Involved and Number of Value of Number Selling Cost of Gain Description of Assets Purchases Purchases (1) of Sales Price (1) Assets Sold (Loss) --------------------- --------- ------------- -------- --------- ----------- ------- Single Transaction Exceeds 5% of Value: None. ------------------------------------------------------------------------------------------------------------------------------ Series of Transactions With Same Broker Exceeds 5% of Value: None. ------------------------------------------------------------------------------------------------------------------------------ Series of Transactions In Same Security Exceeds 5% of Value: RiverSource Trust Money Market Fund I*, f/k/a American Express Trust Money Market Fund I 461 $62,154,696 498 $58,838,114 $58,838,114 $ - ------------------------------------------------------------------------------------------------------------------------------ Single Transaction With One Broker Exceeds 5% of Value: None. ------------------------------------------------------------------------------------------------------------------------------ (1) The purchase/selling price was equal to the fair value on the date of purchase/sale. * Represents party known to be a party-in-interest to the Alliant Energy 401(k) Savings Plan. 18 SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the Total Compensation Committee, which administers the Plan, has duly caused this Annual Report to be signed on its behalf by the undersigned, thereunto duly authorized, on this 28th day of June 2006. ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN /s/ Christopher J. Lindell ---------------------------- Christopher J. Lindell The foregoing person is the Vice President - Shared Services of Alliant Energy Corporation and Alliant Energy Corporate Services, Inc., and the Chairperson of the Alliant Energy Corporation Total Compensation Committee. 19 EXHIBIT INDEX TO ANNUAL REPORT ON FORM 11-K ALLIANT ENERGY CORPORATION 401(k) SAVINGS PLAN FOR THE YEAR ENDED DECEMBER 31, 2005 Page Number in Sequentially Numbered Exhibit No. Exhibit Form 11-K ---------- ------- --------------------- 23 Consent of Independent Registered Public Accounting Firm 21 20