Exhibit 99.1

                            CERTIFICATION PURSUANT TO
                             18 U.S.C. SECTION 1350
                             AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

      In connection with the Quarterly  Report of California Water Service Group
("Company") on Form 10-Q for the period ending  September 30, 2002 as filed with
the Securities  and Exchange  Commission on the date hereof (the  "Report"),  I,
Peter C. Nelson, President and Chief Executive Officer of the Company,  certify,
pursuant to 18 U.S.C.  Section.  1350, as adopted pursuant to Section 906 of the
Sarbanes-Oxley Act of 2002, that:

(1)   The Report fully complies with the  requirements of Section 13(a) or 15(d)
      of the Securities Exchange Act of 1934; and

(2)   The information  contained in the Report fairly presents,  in all material
      respects, the financial condition and results of operations of the Company


                                    /s/ Peter C. Nelson
                                    -------------------
                                    Peter C. Nelson
                                    President and Chief Executive Officer
                                    November 7, 2002