UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 3)*

(Name of Issuer)

(Title of Class of Securities)

(CUSIP Number)

Laurie A. Smiley, Esq.

Arian Colachis, Esq.

2365 Carillon Point

Kirkland, WA 98033

(425) 889-7900

(Name, Address and Telephone Number of Person Authorized to

Receive Notices and Communications)

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box [  ].

Note:     Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See § 240.13d-7 for other parties to whom copies are to be sent.

*The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

(1)All shares of Common Stock held by Cascade Investment, L.L.C. (“Cascade”) may be deemed to be beneficially owned by William H. Gates III as the sole member of Cascade.

2

(1)All shares of Common Stock held by Cascade Investment, L.L.C. (“Cascade”) may be deemed to be beneficially owned by William H. Gates III as the sole member of Cascade.

(2) For purposes of Rule 13d-3 under the Securities Exchange Act of 1934, as amended, all shares of Common Stock beneficially owned by Bill & Melinda Gates Foundation Trust (the “Trust”) may be deemed to be beneficially owned by William H. Gates III and Melinda French Gates as Co-Trustees of the Trust.

3

(1) For purposes of Rule 13d-3 under the Securities Exchange Act of 1934, as amended, all shares of Common Stock beneficially owned by Bill & Melinda Gates Foundation Trust (the “Trust”) may be deemed to be beneficially owned by William H. Gates III and Melinda French Gates as Co-Trustees of the Trust.

4

(1) For purposes of Rule 13d-3 under the Securities Exchange Act of 1934, as amended, all shares of Common Stock beneficially owned by Bill & Melinda Gates Foundation Trust (the “Trust”) may be deemed to be beneficially owned by William H. Gates III and Melinda French Gates as Co-Trustees of the Trust.

5

EXPLANATORY STATEMENT

This Amendment No. 3 to Schedule 13D (“Amendment”) relates to the Common Stock, $1.00 par value per share (the “Common Stock”) of Ecolab Inc. (the “Issuer”).  Cascade Investment, L.L.C. (“Cascade”), Bill & Melinda Gates Foundation Trust (the “Trust”), William H. Gates III and Melinda French Gates (collectively, the “Reporting Persons”) jointly file this Amendment to amend and supplement the Items set forth below of the Reporting Persons’ Schedule 13D previously filed with the Securities and Exchange Commission on July 29, 2011, as amended August 24, 2011 and August 28, 2011.  Neither the present filing nor anything contained herein shall be construed as an admission that the Reporting Persons constitute a “group” for any purpose and the Reporting Persons expressly disclaim membership in a group.

Item 3.                                     Source and Amount of Funds or Other Consideration

On December 8, 2011, in accordance with the merger between the Issuer and Nalco Holding Company (“Nalco”), Cascade received 1,331,967 shares of the Issuer’s Common Stock at an exchange rate of $.7005 per share and an aggregate of approximately $4.68 million for the 2,022,100 shares of Nalco it held.

Item 5.                                     Interest in Securities of the Issuer

(a)     See items 11 and 13 of the cover pages to this Schedule 13D for the aggregate number of shares and percentage of Common Stock beneficially owned by each of the Reporting Persons.

(b)     See items 7 through 10 of the cover pages to this Schedule 13D for the number of shares of Common Stock beneficially owned by each of the Reporting Persons as to which there is sole power to vote or to direct the vote, shared power to vote or to direct the vote and sole or shared power to dispose or to direct the disposition.

(c)     See Item 3 above.

(d)     None.

(e)     Not applicable.

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SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

(1)This Amendment is being filed jointly by the Reporting Persons pursuant to the Joint Filing Agreement dated July 29, 2011 and included with the signature page to the Reporting Persons’ Schedule 13D filed with respect to the Issuer on July 29, 2011, SEC File No. 005-30248 and incorporated by reference herein.

(2) Duly authorized under Special Limited Power of Attorney appointing Alan Heuberger attorney-in-fact, dated August 12, 2008, by and on behalf of Michael Larson, filed as Exhibit 99.1 to Amendment No. 1 to Cascade’s Schedule 13D with respect to Otter Tail Corporation on April 15, 2009, SEC File No. 005-06638 and incorporated by reference herein.

(3) Duly authorized under Special Limited Power of Attorney appointing Alan Heuberger attorney-in-fact, dated August 12, 2008, by and on behalf of William H. Gates III and Melinda French Gates as Co-Trustees, filed as Exhibit 99.5 to Cascade’s Schedule 13D with respect to Grupo Televisa, S.A.B. on May 7, 2009, SEC File No. 005-60431 and incorporated by reference herein.

(4) Duly authorized under Special Limited Power of Attorney appointing Alan Heuberger attorney-in-fact, dated August 12, 2008, by and on behalf of William H. Gates III, filed as Exhibit 99.2 to Amendment No. 1 to Cascade’s Schedule 13D with respect to Otter Tail Corporation on April 15, 2009, SEC File No. 005-06638 and incorporated by reference herein.