EXHIBIT 5.1
 
January 27, 1999
 
Board of Directors of California Water Service Group
c/o Robert W. Foy
Chairman of the Board
California Water Service Group
1720 North First Street
San Jose, CA 95112-4598
 
     Re:  California Water Service Group -- Registration Statement on Form S-4
        Filed on January 28, 1999 Under The Securities Act of 1933, as Amended.
 
Gentlemen:
 
     This opinion is being furnished to you in connection with California Water
Service Group's (the "Corporation") registration statement on Form S-4 filed on
January 28, 1999 (the "Registration Statement") relating to the issuance of
common shares, no par value (the "Shares"), of the Corporation pursuant to the
Agreement and Plan of Reorganization dated as of November 13, 1998 by and among
the Corporation, California Water Service Company and Dominguez Services
Corporation (the "Reorganization Agreement").
 
     You have requested our opinion as to the legality of the shares being
registered, and as to whether the shares will, when sold, be legally issued,
fully paid and non-assessable.
 
     In rendering this opinion, we have (with your permission) relied upon and
assumed as correct and complete the factual representations and information
contained in (i) the articles and by-laws of the Corporation, and (ii) the
Reorganization Agreement.
 
     We have also made such inquiries and examined originals (or copies
certified or otherwise identified to our satisfaction) of documents, corporate
records and other instruments and made such examination of the law as we have
deemed necessary or appropriate to enable us to render this opinion.
 
     We have assumed that the signatures on the documents we have examined are
genuine and further assumed that at the time of issuance of the Shares the
Corporation will have received the full amount of the consideration therefore in
accordance with the terms of the Reorganization Agreement.
 
     Based upon and subject to the foregoing, we are of the opinion that the
Shares have been duly and validly created and authorized and, when issued, will
be validly issued and outstanding as fully paid and non-assessable common shares
of the Corporation.
 
     This opinion is rendered only to the Corporation in connection with the
filing of the Registration Statement and may not be relied upon by any other
person, firm or entity for any other purpose without our express written
consent, and may not be used, circulated, quoted or otherwise referred to for
any other purpose without our express written consent.
 
     We hereby consent to the filing of this letter as Exhibit 5.1 to the
Registration Statement and to being named in the Proxy Statement/Prospectus of
the Registration Statement under the heading "Legal Opinion" as counsel for the
Corporation.
 
                                          Very truly yours,
                                           /s/  NOSSAMAN, GUTHNER, KNOX &
                                          ELLIOTT, LLP
                                          --------------------------------------
                                          Nossaman, Guthner, Knox & Elliott, LLP
 
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