As filed with the Securities and Exchange Commission on September 26, 2006

Registration No. 333-136844

UNITED STATES SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Amendment No. 1

to

Form S-3

REGISTRATION STATEMENT UNDER THE SECURITIES ACT OF 1933

California Water Service Group

(Exact Name of Registrant as Specified in Its Charter)

1720 North First Street, San Jose, CA 95112

(408) 367-8200

(Address, Including Zip Code, and Telephone Number, Including Area Code,

of Registrant’s Principal Executive Offices)

Martin A. Kropelnicki

Vice President, Chief Financial Officer and Treasurer

California Water Service Group

1720 North First Street

San Jose, California 95112

(408) 367-8200

(Name, Address, Including Zip Code, and Telephone Number,

Including Area Code, of Agent For Service)

with a copy to:

Peter T. Heilmann, Esq.

Gibson, Dunn & Crutcher LLP

One Montgomery Street, Suite 3100

San Francisco, California 94104

(415) 393-8200

     Approximate Date of Commencement of Proposed Sale to the Public: From time to time after the effective date of this registration statement.

     If the only securities being registered on this form are being offered pursuant to dividend or interest reinvestment plans, please check the following box.    o

     If any of the securities being registered on this form are to be offered on a delayed or continuous basis pursuant to Rule 415 under the Securities Act of 1933, other than securities offered only in connection with dividend or interest reinvestment plans, check the following box.    þ

     If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

     If this form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.    o

     If this Form is a registration statement pursuant to General Instruction I.D. or a post-effective amendment thereto that shall become effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act, check the following box.    o

     If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box.    o

     The Registrant hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the Registration Statement shall become effective on such date as the Securities and Exchange Commission, acting pursuant to said Section 8(a), may determine.

     Pursuant to Securities Act Rule 429, the prospectus that is a part of this registration statement is a combined prospectus that also relates to Registration Statement on Form S-3 No. 333-103721 (originally filed March 11, 2003, as amended).

Explanatory Note

        This Amendment No. 1 is being filed solely for the purpose of filing Exhibits 1 and 5 to the Registration Statement on Form S-3 (File No. 333-136844). This Amendment No. 1 does not modify any provisions of the prospectus constituting Part I of the Registration Statement. Accordingly, the prospectus has not been included herein.

Part II

Information Not Required in Prospectus

Item 14.      Other Expenses of Issuance and Distribution

       Expenses incurred in connection with the offering of the securities will be borne by the registrant and are estimated as follows:

Item 15.      Indemnification of Directors and Officers

       To the fullest extent permitted by the Delaware General Corporation Law, our certificate of incorporation eliminates a director’s personal liability for monetary damages to the Company and its stockholders, including liability arising from a breach of fiduciary duty as a director. Section 102 of the Delaware General Corporation Law allows a corporation to include in its certificate of incorporation a provision that eliminates the personal liability of the directors of that corporation to the corporation or its stockholders for monetary damages for breach of fiduciary duty as a director, except where the director breached the duty of loyalty, failed to act in good faith, engaged in intentional misconduct or knowingly violated a law, authorized the payment of a dividend or approved a stock repurchase in violation of Delaware corporate law or obtained an improper personal benefit.

      Section 145 of the Delaware General Corporation Law authorizes a court to award, or a corporation’s board of directors to grant, indemnification to directors and officers in terms sufficiently broad to permit such indemnification under certain circumstances for liabilities (including reimbursement for expenses incurred) arising under the Securities Act. Section Seven of the Company’s Certificate of Incorporation provides for indemnification of any person who is or was a party to a proceeding by reason of the fact that such person was an agent of the Company, to the fullest extent permitted under Section 145 of the Delaware General Corporation Law.

      At present, there is no pending litigation or proceeding involving a director, officer, employee or other agent of the Company in which indemnification is being sought, nor are we aware of any threatened litigation that may result in a claim for indemnification by any director, officer, employee or other agent of us.

Item 16.      Exhibits

       See Exhibit Index immediately following the signature page hereof.

Item 17.      Undertakings

       (a) We hereby undertake:

      (b) We hereby undertake that, for purposes of determining any liability under the Securities Act of 1933, each filing of our annual report pursuant to Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934 that is incorporated by reference in this registration statement shall be deemed to be a new registration statement relating to the securities offered herein, and the offering of such securities at that time shall be deemed to be the initial bona fide offering thereof.

      (c) Insofar as indemnification for liabilities arising under the Securities Act of 1933 may be permitted to our directors, officers and controlling persons pursuant to the provisions referred to in Item 15 hereof, or otherwise, we have been advised that in the opinion of the Securities and Exchange Commission such indemnification is against public policy as expressed in the Securities Act and is, therefore, unenforceable. In the event that a claim for indemnification against such liabilities (other than the payment by us of expenses incurred or paid by a director, officer or controlling person of the registrant in the successful defense of any action, suit or proceeding) is asserted by such director, officer or controlling person in connection with the securities being registered, we will, unless in the opinion of our counsel the matter has been settled by controlling precedent, submit to a court of appropriate jurisdiction the question whether such indemnification by us is against public policy as expressed in the Securities Act and will be governed by the final adjudication of such issue.

      (d) We hereby undertake that:

Signatures

        Pursuant to the requirements of the Securities Act of 1933, as amended, the registrant certifies that it has reasonable grounds to believe that it meets all of the requirements for filing on Form S-3 and has duly caused this amendment to registration statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of San Jose, State of California, on September 26, 2006.

      Pursuant to the requirements of the Securities Act of 1933, as amended, this amendment to registration statement has been signed by the following persons in the capacities and on the dates indicated:

Exhibit Index