CALIFORNIA WATER SERVICE GROUP
S-3D, 1998-02-17
WATER SUPPLY
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<PAGE>   1
 
================================================================================
 
                       SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549
 
                            ------------------------
 
                                    FORM S-3
                             REGISTRATION STATEMENT
                                     UNDER
                           THE SECURITIES ACT OF 1933
                            ------------------------
 
                         CALIFORNIA WATER SERVICE GROUP
             (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER)
 
<TABLE>
<S>                               <C>                               <C>
            CALIFORNIA                           4941                           77-0448994
  (STATE OR OTHER JURISDICTION)      (PRIMARY STANDARD INDUSTRIAL            (I.R.S. EMPLOYER
OF INCORPORATION OR ORGANIZATION)    CLASSIFICATION CODE NUMBER)           IDENTIFICATION NO.)
</TABLE>
 
      1720 NORTH FIRST STREET, SAN JOSE, CALIFORNIA 95112, (408) 367-8200
  (ADDRESS, INCLUDING ZIP CODE, AND TELEPHONE NUMBER, INCLUDING AREA CODE, OF
                   REGISTRANT'S PRINCIPAL EXECUTIVE OFFICES)
 
                                PETER C. NELSON
                     PRESIDENT AND CHIEF EXECUTIVE OFFICER
                         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)
 
                            ------------------------
 
                          COPIES OF COMMUNICATIONS TO:
                               WILLIAM J. NEWELL
                     MCCUTCHEN, DOYLE, BROWN & ENERSEN, LLP
                               3150 PORTER DRIVE
                              PALO ALTO, CA 94303
                            650-849-4820 (TELEPHONE)
                               650-849-4800 (FAX)
 
   APPROXIMATE DATE OF COMMENCEMENT OF PROPOSED SALE OF THE SECURITIES 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:  [X]
 
    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.  [ ]
 
    If this Form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following box
and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering.  [ ]
 
    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.  [ ]
 
    If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box.  [ ]
 
                        CALCULATION OF REGISTRATION FEE
 
<TABLE>
<S>                                 <C>               <C>               <C>               <C>
===========================================================================================================
                                                                        PROPOSED MAXIMUM
                                                      PROPOSED MAXIMUM      AGGREGATE
      TITLE OF EACH CLASS OF          AMOUNT TO BE     OFFERING PRICE       OFFERING          AMOUNT OF
    SECURITIES TO BE REGISTERED       REGISTERED(1)     PER SHARE(1)        PRICE(1)      REGISTRATION FEE
- - -----------------------------------------------------------------------------------------------------------
Common Stock, no par value.........  500,000 shares        $29.00          $14,500,000         $4,278
===========================================================================================================
</TABLE>
 
(1) Estimated solely for the purpose of calculating the registration fee based
    on the average of the high and low prices for the Common Stock as reported
    on the New York Stock Exchange on February 13, 1998, pursuant to Rule 457 of
    the Securities Act of 1933, as amended.
 
================================================================================
<PAGE>   2
 
                         CALIFORNIA WATER SERVICE GROUP
                              1720 NORTH FIRST ST.
CWSG                        SAN JOSE, CA 95112-4598                         NYSE
LOGO                             (408) 367-8200                             LOGO
                 DIVIDEND REINVESTMENT AND STOCK PURCHASE PLAN
 
    This Prospectus is for the Dividend Reinvestment and Stock Purchase Plan
(the "new Plan" or the "Plan") of California Water Service Group (the "Group").
The Plan is effective on February 28, 1998. The Plan is the successor Plan to
the California Water Service Company Dividend Reinvestment Plan (the "former
Plan") which terminates on February 28, 1998. Participants' accounts in the
former Plan will automatically transfer to the new Plan as of March 1, 1998, at
which time participants in the former Plan will become participants in the new
Plan without any further action required on their part. Unless a participant in
the former Plan changes the extent of participation as allowed for in the new
Plan, the level of participation in the new Plan will remain the same as in the
former Plan.
 
    The new Plan provides participants with additional and improved features
compared to the former Plan, many of which are described below. The Plan was
developed in connection with the formation, as of December 31, 1997, of a
holding company structure in which the Group became the holding company for
California Water Service Company. The Group's common stock is traded on the New
York Stock Exchange under the symbol "CWT".
 
    The new Plan provides participants with a simple, convenient and economical
method of accumulating shares of the Group's common stock and increasing a
participant's investment in shares of the Group's common stock without incurring
brokerage fees or commissions. In certain instances described in the Plan,
limited fees or service charges may apply. Group common stock holders of record
may participate in the Plan. Minimum investment amounts are $100 with a maximum
of $20,000 in a single calendar month. In addition, any person who is not a
holder of the Group's common stock may become a participant in the Plan by
making an initial investment of not less than $500 and not more than $20,000 per
calendar month. Participants in the Company Employees' Savings Plan can become
Plan Participants by following the procedures outlined for initial investors.
 
    Under the Plan, a participant has the ability to choose between two
reinvestment options:
 
    - The participant may authorize the automatic reinvestment of all cash
      dividends paid on his or her shares of the Group's common stock into
      additional shares of the Group's common stock.
 
    - Alternatively, the participant may authorize the automatic reinvestment of
      only a portion of the cash dividends to be paid on his or her shares into
      additional shares of the Group's common stock, while receiving the balance
      of the dividend in cash.
 
    In addition, the new Plan permits optional direct cash purchases of the
Group's common stock by participants. This feature is available to all
participants, whether they choose full, partial or no dividend reinvestment.
Additional optional cash purchases must be for at least $100, but may not exceed
$20,000 in a single calendar month. The Plan also allows regular, automatic
withdrawals from a participant's bank account with the funds used to make
optional purchases of Group common stock for the participant's account.
 
    At the election of the Group, shares required for the Plan may be either
shares newly issued by the Group or shares acquired by purchases on the open
market. Shares newly issued or acquired through the Plan's dividend reinvestment
feature will be at an average market price as described in the Plan.
 
    The Plan provides participants with the ability to control the level of
their participation in the Plan by allowing a participant to periodically change
the extent to which dividends are reinvested. The Plan also allows for the sale
by a participant of Group common stock through the Plan.
 
    The Plan does not affect the Group's dividend policy, which will continue to
be determined by the Board of Directors after considering the Group's earnings,
financial requirements, operating projections, economic conditions and other
factors.
 
    A more complete description of the Plan is contained in this Prospectus and
reference is hereby made to that description. The Plan should be reviewed
carefully by all participants or prospective participants. Please retain this
Prospectus for future reference.
 
         THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE
        SECURITIES AND EXCHANGE COMMISSION NOR HAS THE COMMISSION PASSED
      UPON THE ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION
                     TO THE CONTRARY IS A CRIMINAL OFFENSE.
 
               The date of this Prospectus is February 17, 1998.    
<PAGE>   3
 
                             ADDITIONAL INFORMATION
 
     The Group is subject to the informational requirements of the Securities
and Exchange Act of 1934 (the "1934 Act") and in accordance therewith submits
reports, proxy statements and other information to the Securities and Exchange
Commission (the "Commission"). Such reports, proxy statements and other
information can be inspected and copied at the public reference facilities of
the Commission at Room 1024, 450 Fifth Street, N.W., Washington, D.C. 20549, and
at the Commission's regional offices in Chicago (Northwestern Atrium Center,
Suite 1400, 500 West Madison Street, Chicago, IL 60661) and New York (Public
Reference Room, 13th Floor, 7 World Trade Center, New York, NY 10048). Copies of
such material can be obtained from the Public Reference Section of the
Commission, 450 Fifth Street, N.W., Washington D.C. 20549, at prescribed rates,
or may be obtained via the Internet through the Commission's EDGAR system. Such
reports, proxy statements and other information can also be inspected at the New
York Stock Exchange, 20 Broad Street, New York, NY 10005, the stock exchange on
which the Common Stock of the Group is listed, and traded under the symbol
"CWT".
 
     The Group intends to continue its present practice of issuing annual
reports to shareholders, containing audited and unaudited financial data.
 
     This Prospectus does not contain all the information set forth in the
Registration Statement and the exhibits relating thereto which the Group has
filed with the Commission under the Securities Act of 1933, as amended, with
respect to the shares of Common Stock offered hereby, and to which reference is
hereby made. The Group will provide without charge to each person to whom this
Prospectus is delivered, upon request, a copy of any or all of the documents
incorporated by reference in this Prospectus. Written requests should be
directed to:
                         California Water Service Group
                         Attention: Corporate Secretary
                            1720 North First Street
                            San Jose, CA 95112-4598
                                 (408) 367-8200
 
                INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE
 
     Effective at the close of business on December 31, 1997, a holding company
structure was adopted of which California Water Service Group is the parent
company. Prior to formation of the holding company, all operations were
conducted by California Water Service Company ("Company"), which upon formation
of the holding company became a subsidiary of the Group. The following
documents, which were filed by Company with the Securities and Exchange
Commission, are hereby incorporated in this Prospectus by reference:
 
          1. The Company's Annual Report on Form 10-K for the year ended
     December 31, 1996.
 
          2. The Company's Quarterly Reports on Form 10-Q for the quarters ended
     March 31, 1997, June 30, 1997 and September 30, 1997.
 
          3. The Group's Registration Statement on Form S-4 No. 333-22915,
     including the Proxy Statement/Prospectus related to the Agreement of Merger
     between Company and California Water Service Group filed March 6, 1997.
 
          4. The Group's current report on Form 8-K, filed January 2, 1998.
 
                                        2
<PAGE>   4
 
     In addition, all documents filed by the Group with the Commission after the
date of this Prospectus pursuant to Sections 13(a), 13(c), 14 and 15(d) of the
1934 Act, and prior to the termination of the offering made hereby, shall be
deemed to be incorporated in this Prospectus by reference and to be a part
hereof from their date of filing.
 
                                   THE GROUP
 
     California Water Service Group, a California corporation, is a holding
company whose subsidiary companies provide, among other things, water service to
about 380,000 customers in 57 California communities through 21 separate water
systems or districts. Service to approximately 375,000 customers is subject to
regulation by the California Public Utilities Commission ("CPUC"). The Group's
subsidiaries also provide utility related services, which are not subject to
CPUC regulation, to other entities including operation and maintenance of
municipal water systems under various contracts.
 
     The Group is the issuer of the shares of its Common Stock, no par value per
share ("Common Stock" or "common shares"), offered under the Dividend
Reinvestment and Stock Purchase Plan of the Group (the "Plan"). The Group's
general mailing address is 1720 North First Street, San Jose, CA 95112-4598, and
its telephone number is (408) 367-8200.
 
     Prior to adoption of the holding company structure of which the Group is
the parent company, all operations were conducted by the Company. Since the
formation of the holding company which was effective at the close of business on
December 31, 1997, Company has been a subsidiary of the Group.
 
     This Plan is the successor to the former Company Dividend Reinvestment Plan
(the "former Company Plan") which was sponsored by Company. The accounts for
Shareholders who participated in the former Company Plan have been automatically
rolled over into the new Plan. No action is required by any shareholder who
participated in the former Company Plan in order to be a Participant, as defined
in this document, in the new Plan.
 
                             THE PLAN ADMINISTRATOR
 
     BankBoston, N.A. (Boston EquiServe) (the "Plan Administrator") is
responsible for administering the Plan. Its duties are described later in this
Prospectus. All communications to the Plan Administrator should be directed to
the following address and telephone number:
 
                       BankBoston, N.A.(Boston EquiServe)
                                 P. O. Box 8040
                             Boston, MA 02266-8040
                                1-(800) 736-3001
 
                             PROVISIONS OF THE PLAN
 
     The following information, which is presented in question-and-answer form,
constitutes the Dividend Reinvestment and Stock Purchase Plan of California
Water Service Group.
 
                                        3
<PAGE>   5
 
PURPOSE AND ADVANTAGES
 
 1. What is the purpose of the Plan?
 
     The purpose of the Plan is to provide record holders of the Group's Common
Stock, including participants in the Company's Employees' Savings Plan, and
other investors interested in becoming Group shareholders with a simple,
convenient and economical method of purchasing and increasing their investment
in shares of the Group's Common Stock. The Plan is designed for all
shareholders, regardless of the size of their holdings.
 
     Under the Plan, the Group has two options by which it may provide shares
required by the Plan. First, the Group may elect to retain funds received from
Participants and issue new common shares. In this manner, the Plan provides the
Group an opportunity to generate additional equity capital for general corporate
purposes. Second, using funds received from Participants, shares may be
purchased on the open market and credited to Plan Participants. The Group, at
its sole discretion, may periodically change from one option to the other.
 
 2. What are the advantages of the Plan to Participants?
 
     A shareholder or other investor who participates in the Plan
("Participant") may exercise the following choices:
 
     - Automatic reinvestment of all or a portion of the cash dividends on
       Common Stock in additional common shares, and the right to make optional
       purchases ("Optional Purchases") by submitting to the Plan Administrator
       cash payments for the purchase of additional Common Shares. If a portion
       of the dividends are reinvested, the balance will be paid to the
       Participant in cash. Limits on the permitted amounts of optional cash
       payments are stated later in this Prospectus. (See Question 18).
 
     - The choice to receive dividends in cash and the right to make Optional
       Purchases by submitting to the Plan Administrator cash payments for the
       purchase of additional common shares. Limits on the permitted amounts of
       optional cash payments are stated later in this Prospectus. (See Question
       18).
 
     - Interested investors may become new Group shareholders by purchasing
       their initial shares of Common Stock through the Plan without paying
       broker fees or commissions. Such purchases make the new shareholders
       eligible to be Participants. A $10 service charge will be retained by the
       Plan Administrator to cover a portion of the cost of establishing a new
       shareholder's account on each first time purchase. Refer to Question 5
       for limits applicable to first time purchases of the Group's Common
       Stock.
 
     - Participants will receive periodic statements reporting their purchases
       of Common Stock, thus simplifying record-keeping.
 
     - A Participant may wish to make regular monthly Optional Purchases, and
       can do so by authorizing an automatic withdrawal from his or her bank
       account. (See Question 17).
 
     - The Plan allows a Participant flexibility in the amount of investments he
       or she wishes to make and the manner in which the investments are to be
       made. A Participant may choose to have Automatic Purchases made with all
       or a portion of his or her dividends, may make Optional Purchases in any
       amount, subject to the limitations stated under Question 18, and may
       cease or vary the amounts of such purchases from time to time.
 
                                        4
<PAGE>   6
 
     - A Participant may submit certificates of Group Common Stock to the Plan
       Administrator so that such shares will be added to the Participant's Plan
       Account. Certificates submitted to the Plan Administrator will be
       canceled and the Participant's Plan Shares increased accordingly.
 
     - Shares held in the Plan may at the direction of the Participant be sold
       by the Plan Administrator as explained in Question 22.
 
PARTICIPATION
 
 3. Who is eligible to participate in the Plan?
 
     (a) Shareholders of record of the Group's Common Stock are eligible to
participate in the Plan.
 
     (b) Shareholders whose common shares are held in brokerage, bank or other
intermediary accounts, commonly referred to as "street name registration," can
become Participants by instructing their broker, bank or trustee to register
some or all of their Group shares directly in the shareholder's name, then
completing and returning an authorization form ("Authorization Form") to the
Plan Administrator. Participants in the Company Employees Savings Plan can also
become Participants by establishing a Plan Account as described in Question 5.
 
     (c) Other interested investors who do not currently own Group Common Stock.
(Refer to Question 5).
 
 4. How does an eligible shareholder participate?
 
     Any shareholder of record may become a Plan Participant and join the Plan
by completing an Authorization Form and returning it to the Plan Administrator.
An Authorization Form may be obtained by calling, 1-800-736-3001 or writing the
Plan Administrator identified on Page 4. Refer to Question 18 for purchase
dollar amount limits. First time purchasers of Group Common Stock should refer
to Question 5.
 
 5. How does an interested investor participate in the Plan?
 
     An interested investor or participants in the Company's Savings Plan may
join the Plan at any time by completing an enrollment form ("Enrollment Form")
and returning it to the Plan Administrator. Enrollment Forms may be obtained by
request to the Plan Administrator whose telephone number is 1-800-337-3503. The
interested investor need not be a registered holder of Common Stock, but by
executing the Enrollment Form, agrees to make a first time purchase of Common
Stock of not less than $500, nor more than $20,000, purchased on their behalf on
the next Investment Day, as defined under Question 17, at a price equal to the
applicable average market price, as described under Question 15. Each Enrollment
Form for an interested investor must be accompanied by a check or money order,
made payable to "BankBoston, N. A., Plan Administrator" for a minimum of $500. A
$10 service charge will be retained by the Plan Administrator to cover a portion
of the cost of establishing a new shareholder account on each first time
purchase.
 
 6. When may an eligible person join the Plan?
 
     Anyone eligible to become a Participant may join the Plan at any time.
However, the Authorization Form or Enrollment Form must be received by the Plan
Administrator before certain deadlines in order for the dividends to be invested
by a specific dividend date. (See Question 13).
 
                                        5
<PAGE>   7
 
 7. How is the Authorization Form used?
 
     The Authorization Form is used to instruct the Plan Administrator to open
an account for a Participant ("Plan Account") and to acquire Common Stock on the
Participant's behalf in accordance with the Plan. A Participant must furnish a
federal tax identification number to the Plan Administrator before a Plan
Account will be activated.
 
 8. How will Common Stock be acquired for a Participant's Account?
 
     At the Group's discretion, either of two options, which are outlined below,
may be selected. The option selected may be changed periodically at the Group's
sole discretion.
 
     First, the Plan Administrator can purchase Common Stock on the
Participant's behalf using the Participant's Common Stock dividends designated
for this purpose, or using optional payments received from Participants, subject
to the limitations stated under Question 18. Second, in lieu of purchasing
shares on the open market, the Plan also allows for the issuance of new shares
of Common Stock to satisfy the requirements of the Plan. (See Question 1).
 
     All shares of Common Stock which the Plan Administrator acquires for a
Participant under the Plan, whether through the automatic reinvestment of
dividends or with optional payments, will be credited to the Participant's Plan
Account and held on his or her behalf by the Plan Administrator. Thus, the
shares credited to a Participant under the Plan will be held separately from the
shares of Common Stock which the Participant purchases or has previously
purchased outside the Plan and holds in his or her own name, or in street name.
 
 9. How does a Participant specify the extent of his or her participation in the
Plan?
 
     A Participant will specify on the Authorization Form or Enrollment Form the
extent of his or her participation in the Plan by selecting one of the following
investment options:
 
     Full Dividend Reinvestment -- Dividends on all shares of Common Stock held
     by the Participant in the Plan or held in the Participant's own name
     outside of the Plan will be automatically reinvested; thus, the dividends
     on all such shares will automatically be reinvested in Common Stock at a
     price equal to the applicable average market price. In addition, the
     Participant may, at his or her discretion, make optional payments to be
     used for Optional Purchases of Common Stock at a price equal to the
     applicable average market price (see Question 15), subject to the
     limitations stated under Question 18.
 
     Partial Dividend Reinvestment -- Except shares for which the Participant
     has specified he or she is to receive cash dividends, all of the shares of
     Common Stock held by the Participant in the Plan or outside the Plan will
     be subject to automatic dividend reinvestment; thus, the dividends on all
     but the specified shares will automatically be reinvested in Common Stock
     at a price equal to the applicable average market price. The Participant
     may also, at his or her discretion, make optional payments to be used for
     Optional Purchases of Common Stock at a price equal to the applicable
     average market price (see Question 15), subject to the limitations stated
     under Question 18.
 
     Optional Purchases Only -- None of the shares of Common Stock held by the
     Participant in the Plan or outside of the Plan will be subject to automatic
     dividend reinvestment; thus, the dividends on all shares will be paid to
     the Participant in cash. However, the Participant may, at his or her
     discretion, make Optional Payments to be used for Optional Purchases of
     Common Stock at a price equal to the applicable average market price (See
     Question 15), subject to the limitations stated under Question 18.
 
                                        6
<PAGE>   8
 
10.May a Participant change the extent of the participation in the Plan after
   enrollment?
 
     Yes, a Participant may change the extent of participation by changing
options at any time by completing a new Authorization Form and returning it to
the Plan Administrator. However, the new Authorization Form must be received
before certain deadlines in order for the change to be effective before the next
dividend reinvestment date. (See Question 13).
 
11. How will certificates for shares acquired under the Plan be issued?
 
     Normally, certificates for shares of Common Stock acquired under the Plan
will not be issued to Participants. Thus, Participants need not be responsible
for the safekeeping of the certificates representing their Plan shares. Shares
acquired for a Participant will be credited to each Participant's Plan Account
and will be shown on Plan statements. (See Question 20).
 
     A Participant may request that all or some of the certificates representing
his or her shares purchased under the Plan be issued in certificate form. To do
so, a Participant must provide the Plan Administrator with appropriate
instructions by telephone or in writing. Only certificates for whole shares will
be issued. Certificates for fractional shares held in a Participant's Plan
Account will not be issued. Dividends on those shares for which certificates are
issued to the Participant will be reinvested or paid in cash, as the Participant
elects. (Refer to Question 9).
 
     New shares issued as a result of a stock split will be credited to
Participants' accounts.
 
ADMINISTRATION
 
12. What are the duties of the Plan Administrator?
 
     The Plan Administrator will establish and maintain a Plan Account ("Plan
Account") for each Participant, will cause all purchases of shares of Common
Stock to be made for each Participant, will credit those purchases to the
Participant's Plan Account, will keep a record of all such purchases, will hold
the Participant's Plan shares unless otherwise instructed in writing, will
accept shares which were previously held by the Participant outside of the Plan
into the Participant's Plan Account, and will send each Participant periodic
statements of the Plan Account.
 
13. When will dividends be paid?
 
     Quarterly dividends are expected to be paid on dividend payment dates as
determined by action of the Group's Board of Directors. Based on historical
dividend payment dates, future dividend payment dates for the Group's Common
Stock are expected to occur on or about each February 15, May 15, August 15 and
November 15.
 
     The dividend record dates corresponding to those dividend payment dates
generally are expected to be between fifteen to twenty-one days prior to the
dividend payment dates. To provide for automatic dividend reinvestment of a
given dividend payment, a Participant's Authorization Form or Enrollment Form
must be received on or before the dividend record date. If an Authorization Form
or Enrollment Form is received after the dividend record date, the pending
dividend may be paid to the shareholder in cash and, in such event, the
instructions in the Authorization Form or Enrollment Form will be given effect
starting with the next dividend payment.
 
                                        7
<PAGE>   9
 
14. How will Automatic Purchases be made?
 
     Shares to be acquired through Automatic Purchases under the Plan will be
either new shares of Common Stock issued directly by the Group or shares
purchased on the open market (see Question 1). The number of shares to be
purchased for each Participant through an Automatic Purchase will depend upon
the dollar amount of the dividends being reinvested and the applicable average
market price of the Common Stock (see Question 15). The Plan Administrator will
purchase as many whole shares and fractional shares, computed to four decimal
places, as can be purchased with the available dividends.
 
15. How will the price of shares purchased through Automatic Purchases be
determined?
 
     If the shares acquired by the Plan are shares newly issued by the Group,
the applicable average market price will be the average of the high and low
sales prices for the Common Stock for each of the last five days on which the
Common Stock was traded prior to the dividend payment date, as published in The
Wall Street Journal New York Stock Exchange composite transactions list.
 
     If the shares acquired by the Plan are purchased on the open market, the
applicable average market price will be the weighted average price of all shares
so purchased.
 
16. Will shares acquired through Automatic Purchases be subject to automatic
    dividend reinvestment?
 
     Yes. All dividends paid on shares acquired through Automatic Purchases will
automatically be reinvested in additional shares of Common Stock, unless the
Participant elects to receive all or a portion of his or her dividend in cash.
If certificates for shares acquired through Automatic Purchases are issued to
the Participant, the dividends paid on such shares will continue to be
reinvested unless the Participant elects to have them paid in cash by changing
his or her extent of participation (see Questions 9 and 10).
 
17. When may Optional Purchases be made?
 
     A Participant may initiate an Optional Purchase when enrolling in the Plan
by enclosing an optional payment (a check or money order payable to "BankBoston,
N. A., Plan Administrator") with the Authorization Form and returning it to the
Plan Administrator. The Optional Payment will be invested in shares of Common
Stock on the next "Investment Day" as described below. Payments received without
an Authorization Form or with an incomplete Authorization Form will be returned.
After initial enrollment in the Plan, a Participant may make Optional Purchases
as often as weekly by sending an optional payment with an Optional Purchase Form
to the Plan Administrator. A $25.00 fee will be assessed to the Participant for
a check that is returned for insufficient funds. Refer to Questions 18 for
Optional Purchase limits.
 
     Additionally, a Participant may wish to make regular monthly optional
payments, and can do so by authorizing an automatic withdrawal from his or her
bank account by completing an automatic monthly investment form ("Automatic
Optional Payment") form and returning it along with a canceled check or deposit
slip to the Plan Administrator. A $25.00 fee will be assessed to the Participant
for an optional payment which is rejected by the Participant's bank due to
insufficient funds. Optional payments made through Automatic Optional Payment
will be invested on a monthly basis and Participants' bank accounts will be
debited on the first banking day of the month. The funds will be invested on the
Investment Day in the week following the debit.
 
     Optional Payments received by the Plan Administrator will be invested in
shares of Common Stock at the weighted average price of all shares purchased.
Purchases will be made once each week on the Investment Day for that week, which
will be each Wednesday, or the next business day, if Wednesday is a bank
holiday.
 
                                        8
<PAGE>   10
 
Notwithstanding any other provisions of this Plan, the Group has, however,
instructed the Plan Administrator that purchases of Common Shares for the Plan
may be spread over several days in order to lessen any impact on the Common
Stock price due to purchases on behalf of the Plan. No interest will be paid to
any Participant's Optional Payments between the time the Plan Administrator
receives them and the time they are invested.
 
     The last day that the Plan Administrator will accept Optional Payments for
a given week is the close of business two banking days prior to the Investment
Day. For example, when the Investment Day is a Wednesday, an Optional Payment
must be received by Monday to be included in the week's purchase. If Monday were
a bank holiday, the Optional Payment must be received by the previous Friday.
Any payments received less than two banking days prior to the Investment Day
will be invested on the Investment Day for the following week.
 
     If the shares acquired by the Plan are shares newly issued by the Group,
the applicable average market price will be the average of the high and low
sales prices for the Common Stock for each of the last five days on which the
Common Stock was traded prior to the dividend payment date, as published in The
Wall Street Journal New York Stock Exchange composite transactions list.
 
     If the shares acquired by the Plan are purchased on the open market, the
applicable average market price will be the weighted average price of all shares
so purchased.
 
     If a Participant submits an Optional Payment, and then wishes to have it
returned rather than invested, the Participant must notify the Plan
Administrator in writing of such request. The Plan Administrator may comply with
such request at its discretion.
 
18. In what amounts may Participants make Optional Purchases?
 
     The amount of Optional Purchases may vary from time to time at the
Participant's discretion, and Participants are under no obligation at any time
to make an Optional Purchase. The minimum Optional Purchase per Participant is
$100 with a maximum of $20,000 per calendar month. Optional Payments not
satisfying the stated limits will be returned to the Participant. For limitation
purposes, all Plan Accounts under common control or management will be
aggregated and deemed to be one account. The full amount of any week's Optional
Purchases for a Plan Account must be submitted to the Plan Administrator in a
single payment.
 
     The Company Employees' Savings Plan operates independent of this Plan. If
an employee who owns Group Common Stock in the Company Employees' Savings Plan
participates in this Plan, a separate Plan Account under the Dividend
Reinvestment and Stock Purchase Plan will be established for the employee. Refer
to Question 5 for details of how to become a Participant in this Plan.
 
     Optional Payments received from foreign Participants must be in United
States dollars and will be invested in the same way as Optional Payments from
other Participants.
 
COSTS
 
19. Are any fees or expenses incurred by a Participant in the Plan?
 
     Participants will incur no broker commissions or administrative charges for
purchases made through the Plan, except as noted in Question 2. There may be
certain charges incurred upon a Participant's withdrawal from the Plan, which
are described under Question 22. Other costs of maintaining the Plan will be
borne by the Group.
 
                                        9
<PAGE>   11
 
STATEMENTS AND REPORTS TO PARTICIPANTS
 
20. What type of statements and reports will be sent to Participants?
 
     Each Participant will receive a statement after each transaction in the
Plan Account, which will reflect the year-to-date activity and the share balance
in the Plan Account. Participants will also receive the same communications as
other shareholders, including annual and quarterly shareholder reports, and
proxy statements.
 
WITHDRAWAL AND TERMINATION
 
21. When and how may a Participant withdraw from the Plan?
 
     A Participant may withdraw from the Plan at any time by submitting a
written request for withdrawal to the Plan Administrator. A Participant who
withdraws from the Plan may not rejoin the Plan for a 12-month period. The lower
portion of the account statement may be used to request withdrawal.
 
22. What happens when a Participant withdraws from the Plan?
 
     When a Participant withdraws from the Plan, the Participant will be issued
a certificate representing all of the whole shares credited to the Plan Account,
and the Participant will receive a cash payment in an amount equal to the value
of any fraction of a share of Common Stock remaining in the Plan Account.
 
     If a Participant's request to withdraw from the Plan is received at least
five business days prior to a dividend record date, the withdrawal will be
processed before the close of business on the record date and the Participant
will receive subsequent dividends in cash. If the request to withdraw is
received less than five business days prior to the dividend record date, the
dividend will be invested in Common Stock in accordance with the Participant's
instructions on the last Authorization Form and the request for withdrawal will
then be processed to be effective for the following dividend payment date.
 
     If any optional payments are being held on a Participant's behalf at the
time the request for withdrawal is received, the Plan Administrator will not be
required to return them unless that request is received no later than the fifth
business day prior to the next Investment Day. If the request is received after
the fifth business day prior to the next Investment Day, the optional payments
will be invested in Common Stock and the request for withdrawal will then be
processed.
 
     Upon tendering notice of withdrawal from the Plan, a Participant may
request that all whole shares credited to the Plan Account be sold. The sale
will be made as soon as practicable after receipt of the request. The
Participant will receive the proceeds of the sale, less a brokerage commission,
and a 5% administration fee charged by the Plan Administrator totaling a minimum
of $10 and a maximum of $15.
 
     Participants may also instruct the Plan Administrator to withdraw and sell
a portion of the Participant's Plan shares. The minimum number of shares which
may be sold in this manner is 100. The Participant will receive the proceeds of
the sale, less a brokerage commission, and a 5% administration fee charged by
the Plan Administrator totaling a minimum of $10 and a maximum of $15. If a
Participant has fewer than 100 shares in the Plan and desires to make a sale
through the Plan, all shares must be sold as set forth in the preceding
paragraph.
 
                                       10
<PAGE>   12
 
23. May a Participant discontinue dividend reinvestment on shares held outside
    the Plan without withdrawing from the Plan?
 
     Yes, a Participant who wishes to discontinue the automatic reinvestment of
dividends for shares held outside the Plan may do so, without withdrawing from
the Plan, by filing a new Authorization Form. Dividends on the shares held in
the Plan Account will continue to be reinvested unless the Participant instructs
the Plan Administrator to the contrary.
 
24. What happens if a Participant sells a portion or all of the shares of Common
    Stock held outside the Plan?
 
     If a Participant sells all of the shares of Common Stock held outside the
Plan, the Group will continue to reinvest the dividends on the shares held in
the Plan Account, unless instructed otherwise in writing on an Authorization
Form. However, if less than one whole share is held in the Plan Account at the
time the shares held outside the Plan are sold, the Participant will receive a
cash payment for the fractional share, and the Plan Account will be closed.
 
     If a Participant who has chosen partial dividend reinvestment sells a
portion of the shares of Common Stock held outside the Plan, the shares which
are sold will be considered, to the extent possible, to have been those not
subject to automatic dividend reinvestment. The shares which are retained will
be considered, to the extent possible, to have been those which participated in
dividend reinvestment and will continue to participate in the Plan.
 
25. What happens if the Group terminates the Plan?
 
     If the Group terminates the Plan, the provisions listed under Question 22
above will apply, substituting the termination date of the Plan for the date the
Participant's withdrawal request is received.
 
RIGHTS OFFERINGS AND SHARE DISTRIBUTIONS
 
26. What happens if the Group makes a rights offering or share distribution?
 
     In the event the Group makes a rights offering of any of its securities to
shareholders of Common Stock, other than any rights offering made by the Group
in accordance with any shareholders rights plan or any successor or similar
plan, the Plan Administrator will promptly sell on the open market, to the
extent it is able to do so, the rights attributable to all of the shares held in
Participants' Plan Accounts. The Plan Administrator will then credit each
Participant's Plan Account with the proportionate share of the proceeds of that
sale, and those proceeds will be invested as optional payments on the next
Investment Day. All Participants will be notified by the Group of any such
rights offering. Therefore, any Participant who wishes to exercise the rights
will be required to instruct the Plan Administrator to withdraw the
Participant's Plan shares from the Plan prior to the record date for the rights
distributions.
 
     Any dividend payable in Common Stock or any split shares, to the extent
attributable to shares whether held in a Participant's Plan Account, or to
shares held by a Participant outside the Plan, will be added to the
Participant's Plan Account and thus become Plan shares on which dividends will
be reinvested.
 
TAXES
 
27. What are the federal income tax consequences of participation in the Plan?
 
     The Group believes that the federal income tax consequences of
participating in the Plan will be as follows:
 
                                       11
<PAGE>   13
 
     - Participants who elect automatic dividend reinvestment will be treated
       for federal income tax purposes as having received, on the dividend
       payment date, a cash dividend in an amount equal to the fair market value
       of the shares acquired from the Group with reinvested dividends. Refer to
       Question 15 as to determination of the cost basis of shares issued or
       acquired under the Plan.
 
     - The purchase price of Automatic Purchases and Optional Purchases,
       determined as set forth in Question 15, will be the tax basis for
       determining gain or loss upon any subsequent sale of shares.
 
     - A Participant's holding period for shares acquired through the Plan will
       begin on the day following the credit of such shares to the Participant's
       account. (See Questions 13 and 17).
 
     - Participants who elect automatic dividend reinvestment will receive from
       the Plan Administrator annual information regarding taxable dividends on
       Internal Revenue Service (IRS) form 1099.
 
     - In the case of Participants who elect to have their dividends reinvested
       and whose dividends are subject to United States income tax withholding,
       the Plan Administrator will reinvest an amount equal to the dividends of
       such Participants, less the amount of tax required to be withheld. The
       statements confirming purchases made for such Participants will indicate
       the net dividend payment reinvested.
 
     - Participants who receive cash in lieu of fractional shares and
       Participants whose shares are sold by the Plan Administrator will
       recognize gain or loss on such shares, measured by the difference between
       the selling price of the shares and the Participant's tax basis.
 
28. What information will be provided to Participants for income tax purposes?
 
     The Plan Administrator will provide each Participant with statements for
transactions occurring in the Plan Account, and annual IRS form 1099s. These
statements should be retained for income tax purposes. The Group may impose a
charge to recreate statements.
 
29. Should Participants consult with their own tax advisors?
 
     Yes. Participants should consult with their own tax advisors for additional
information regarding the federal, state and local tax consequences of
participation in the Plan.
 
OTHER INFORMATION
 
30. How will a Participant's shares held under the Plan be voted on matters
    presented to shareholders?
 
     Each Participant's Plan Shares will be voted as the Participant directs.
Participants will receive proxy materials in advance of shareholder meetings and
may give instructions on the voting of his or her Plan shares. If no
instructions are received, the Plan Shares will not be voted.
 
31. May shares held in a Participant's Plan Account be pledged or assigned?
 
     Shares credited to a Participant's Plan Account may not be pledged or
assigned, and any such purported pledge or assignment will be void. If a
Participant wishes to pledge or assign such shares, a certificate for such
shares must first be issued in the Participant's name.
 
                                       12
<PAGE>   14
 
32. Who interprets and regulates the Plan?
 
     The Group reserves the sole right to interpret and regulate the Plan,
including the right to return or reject payments received from Participants or
prospective Participants which the Group determines to be in violation of the
Plan's terms or intent.
 
33. May the Plan be terminated, suspended or amended?
 
     The Group may, in its sole discretion and by written notice, terminate at
any time any Participant's participation in the Plan. The Group may at any time
and for any reason terminate or suspend the Plan, or amend any provision of the
Plan, and if it does so, it will send written notice to all Participants. All
notices will be mailed to each Participant's address as shown on the Group's
records. BankBoston, N. A. (Boston EquiServe) reserves the right to resign as
Plan Administrator, and the Group reserves the right to appoint a successor. The
Group also reserves the right to discharge the Plan Administrator and the right
to appoint a successor.
 
34. What are the responsibilities of the Group and the Plan Administrator?
 
     In acting under the terms and conditions of the Plan as described in this
Prospectus, neither the Group nor the Plan Administrator shall be liable for any
act performed in good faith nor for any good faith omission to act including,
without limitation, any failure, prior to receipt by the Plan Administrator of
notice in writing of the death of a Participant, to terminate a Plan Account by
reason of such death. In addition, neither the Group nor the Plan Administrator
shall be liable with respect to the prices at which shares are purchased or sold
for any Participant's Plan Account or the times when such purchases or sales are
made or with respect to any fluctuation in the market value before or after such
purchases or sales of shares.
 
                                       13
<PAGE>   15
 
                        MARKET PRICE RANGE AND DIVIDENDS
 
     The Group's Common Stock is traded on the New York Stock Exchange under the
symbol "CWT". The following table shows the high and low sale prices per share
of Common Stock, as published in The Wall Street Journal report of New York
Stock Exchange composite transactions, and dividends declared per share, for the
periods indicated. The data presented is for the Group's predecessor, California
Water Service Company and has been restated for the effective two-for-one stock
split that was completed December 31, 1997. (See Page 3, under the caption "THE
GROUP").
 
<TABLE>
<CAPTION>
                    PRICE RANGE
                    ------------      DIVIDENDS
YEAR     QUARTER    HIGH     LOW      DECLARED
- - -----    -------    ----     ---      ---------
<S>      <C>        <C>      <C>      <C>
1995     First      $16 3/16 $14 13/16  $.255
         Second      16 5/16  14  7/8    .255
         Third       16 7/16  14 13/16   .255
         Fourth      17 5/8   16  3/16   .255
 
1996     First       18 5/8   16  1/4    .26
         Second      17 13/16 16  3/4    .26
         Third       19 1/8   16  1/4    .26
         Fourth      21 7/8   17  15/16  .26
 
1997     First       22 5/8   19  1/2    .26375
         Second      23 7/8   18  5/8    .26375
         Third       25 7/32  21  1/32   .26375
         Fourth      29 19/32 23  7/16   .26375
</TABLE>
 
                                USE OF PROCEEDS
 
     The net proceeds from the sale of Common Stock by the Group under the Plan
will be added to the general funds of the Group and used for general corporate
purposes, including but not limited to investment in its subsidiaries.
 
                                 LEGAL OPINION
 
     Legal matters in connection with the authorization and issuance of the
shares of Common Stock offered hereby, and the federal income tax consequences
of participation in the Plan (discussed under Question 27), have been passed
upon by McCutchen, Doyle, Brown & Enersen, LLP, Palo Alto, California.
 
                                    EXPERTS
 
     The financial statements and schedule of California Water Service Company
as of December 31, 1996 and 1995 and for each of the years in the three-year
period ended December 31, 1996, have been incorporated by reference herein and
in the Prospectus in reliance upon the report of KPMG Peat Marwick LLP,
independent certified public accountants, incorporated by reference herein, and
upon the authority of said firm as experts in accounting and auditing.
 
                                       14
<PAGE>   16
 
                   INDEMNIFICATION OF DIRECTORS AND OFFICERS
 
     As authorized by Section 317 of the California General Corporation Law,
Article VI of the Group's Restated Articles of Incorporation provides that the
Group shall indemnify any person who is a party to any suit or proceeding,
whether civil, criminal or administrative, because such person is or was a
director, officer or employee of the Group or another corporation or enterprise,
including an employee benefit plan, against expenses including attorneys' fees,
judgments, fines and amounts paid in settlement actually and reasonably incurred
by such person in connection with such suit or proceeding to the fullest extent
that such indemnification is permitted by applicable law. Pursuant to
resolutions of the Board of Directors of the Group, the Board is required to
appoint an appropriate fact finder to determine in each case whether or not the
standards of conduct permitting indemnification in the applicable statute have
been met.
 
     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers or persons controlling the Group
pursuant to the foregoing provisions, the Group has been informed that in the
opinion of the Securities and Exchange Commission such indemnification is
against public policy as expressed in the Act and is therefore unenforceable.
 
                                       15
<PAGE>   17
 
======================================================
 
     NO DEALER, SALESMAN OR OTHER PERSON HAS BEEN AUTHORIZED TO GIVE ANY
INFORMATION OR TO MAKE ANY REPRESENTATION NOT CONTAINED IN THIS PROSPECTUS AND,
IF GIVEN OR MADE, SUCH INFORMATION OR REPRESENTATION MUST NOT BE RELIED UPON AS
HAVING BEEN AUTHORIZED BY THE GROUP. THIS PROSPECTUS DOES NOT CONSTITUTE AN
OFFER TO SELL OR A SOLICITATION OF AN OFFER TO BUY ANY OF THE SECURITIES OFFERED
HEREBY IN ANY JURISDICTION TO ANY PERSON TO WHOM IT IS UNLAWFUL TO MAKE SUCH
OFFER IN SUCH JURISDICTION.
 
                            ------------------------
 
                               TABLE OF CONTENTS
 
<TABLE>
<CAPTION>
                                        PAGE
                                        ----
<S>                                     <C>
Dividend Reinvestment and Stock
  Purchase Plan.......................    1
Additional Information................    2
Incorporation of Certain Documents by
  Reference...........................    2
The Group.............................    3
The Plan Administrator................    3
Provisions of the Plan................    3
  Purpose and Advantages..............    4
  Participation.......................    5
  Administration......................    7
  Costs...............................    9
  Statements and Reports to
     Participants.....................   10
  Withdrawal and Termination..........   10
  Rights Offerings and Share
     Distributions....................   11
  Taxes...............................   11
  Other Information...................   12
Market Price Range and Dividends......   14
Use of Proceeds.......................   14
Legal Opinion.........................   14
Experts...............................   14
Indemnification of Directors and
  Officers............................   15
</TABLE>
 
======================================================
======================================================
                                CALIFORNIA WATER
                                 SERVICE GROUP
 
                                      LOGO
                                  COMMON STOCK
                                 (NO PAR VALUE)
 
                                   PROSPECTUS
                             DIVIDEND REINVESTMENT
                            AND STOCK PURCHASE PLAN
                               FEBRUARY 17, 1998
 
======================================================
<PAGE>   18
 
                                    PART II
 
                     INFORMATION NOT REQUIRED IN PROSPECTUS
 
ITEM 14. OTHER EXPENSES OF ISSUANCE AND DISTRIBUTION
 
     The following table sets forth all expenses payable by the Registrant in
connection with the sale of the Common Stock being registered. All the amounts
are estimates except for the SEC registration fee.
 
<TABLE>
        <S>                                                                  <C>
        SEC registration fee...............................................  $ 4,278
        Stock exchange listing fees........................................     None
        Registrar and transfer agent fees..................................   10,000
        Printing and distribution..........................................   10,000
        Legal fees and expenses............................................   25,000
        Accounting fees and expenses.......................................    7,500
                                                                             -------
                  Total....................................................  $56,778
                                                                             =======
</TABLE>
 
ITEM 15. INDEMNIFICATION OF OFFICERS AND DIRECTORS.
 
     Section 317 of the California General Corporation Law permits
indemnification of directors, officers and employees of corporations under
certain conditions and subject to certain limitations. Article Ninth of the
Restated Articles of Incorporation, as amended, of the registrant contains
provisions limiting the monetary liability of directors for breaches of the duty
of care. California law does not permit the elimination of liability (i) for any
breach of a director's duty of loyalty to the Company or its shareholders, (ii)
for acts or omissions not in good faith or which involve intentional misconduct
or a knowing violation of law, (iii) in respect of certain unlawful dividend
payments or stock redemptions or repurchases, or (iv) for any transaction from
which the director derives an improper personal benefit. The effect of the
provision in Article Ninth of the Restated Articles of Incorporation is to
eliminate the rights of the Company and its shareholders (through shareholders'
derivative suits on behalf of the Company) to recover monetary damages against a
director for breach of fiduciary duty as a director (including breaches
resulting from negligent or grossly negligent behavior) except in the situations
described in clauses (i)-(iv) above. These provisions will not alter the
liability of directors under the federal securities laws.
 
     Article Ninth of the Restated Articles of Incorporation, as amended, of the
registrant contains provisions for the indemnification of directors, officers
and employees to the fullest extent permitted under Section 317. The Company's
Board of Directors has also adopted resolutions specifying the procedures to be
followed by an officer or director who is seeking indemnification. Such
procedures provide for, among other things, the advancement of expenses by the
Company to the officer or director upon request and upon receipt by the Company
of an undertaking to repay such advance in certain circumstances. In addition,
the Company maintains officers and directors liability insurance for an annual
aggregate maximum coverage of $20,000,000.
 
ITEM 16. EXHIBITS AND FINANCIAL STATEMENT SCHEDULES.
 
     (a) Exhibits.
 
<TABLE>
<CAPTION>
    EXHIBITS                                DESCRIPTION OF EXHIBITS
    --------     ------------------------------------------------------------------------------
    <S>          <C>
     5.1         Form of opinion of McCutchen, Doyle, Brown & Enersen, LLP.
    23.1         Consent of KPMG Peat Marwick LLP.
    23.2         Consent of McCutchen, Doyle, Brown & Enersen, LLP (included in their opinion
                 filed as Exhibit 5).
    24.1         Power of Attorney (included in signature page filed in Part II).
</TABLE>
 
                                      II-1
<PAGE>   19
 
     (b) Financial Statement Schedules.
 
     Not applicable.
 
ITEM 17. UNDERTAKINGS.
 
     1. The undersigned registrant hereby undertakes:
 
          (1) To file, during any period in which offers or sales are being
     made, a post-effective amendment to this registration statement:
 
             (i) To include any prospectus required by Section 10(a)(3) of the
        Securities Act of 1933;
 
             (ii) To reflect in the prospectus any facts or events arising after
        the effective date of the registration statement (or the most recent
        post-effective amendment thereof) which, individually or in the
        aggregate, represent a fundamental change in the information set forth
        in the registration statement. Notwithstanding the foregoing, any
        increase or decrease in volume of securities offered (if the total
        dollar value of securities offered would not exceed that which was
        registered) and any deviation from the low or high and of the estimated
        maximum offering range may be reflected in the form of prospectus filed
        with the Commission pursuant to Rule 424(b) if, in the aggregate, the
        changes in volume and price represent no more than 20 percent change in
        the maximum aggregate offering price set forth in the "Calculation of
        Registration Fee" table in the effective registration statement; and
 
             (iii) To include any material information with respect to the plan
        of distribution not previously disclosed in the registration statement
        or any material change to such information in the registration
        statement;
 
             provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) herein
        do not apply if the information required to be included in a
        post-effective amendment by those paragraphs is contained in periodic
        reports filed with or furnished to the Commission by the registrant
        pursuant to Section 13 or Section 15(d) of the Securities Exchange Act
        of 1934 that are incorporated by reference in the registration
        statement;
 
          (2) That, for the purpose of determining any liability under the
     Securities Act of 1933, each such post-effective amendment shall be deemed
     to be a new registration statement relating to the securities offered
     therein, and the offering of such securities at that time shall be deemed
     to be the initial bona fide offering thereof; and
 
          (3) To remove from registration by means of a post-effective amendment
     any of the securities being registered which remain unsold at the
     termination of the offering.
 
     2. The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to Section 13(a) or 15(d) of the Securities
Exchange Act of 1934 that is incorporated by reference in the Registration
Statement shall be deemed to be a new registration statement relating to the
securities offered therein, and the offering of such securities at that time
shall be deemed to be the initial bona fide offering thereof.
 
                                      II-2
<PAGE>   20
 
                                   SIGNATURES
 
     Pursuant to the requirements of the Securities Act of 1933, 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 caused this registration statement
to be signed on its behalf by the undersigned, thereunto duly authorized in the
City of San Jose, State of California, on February 17, 1998.
 
                                          CALIFORNIA WATER SERVICE GROUP
                                          (Registrant)
 
                                          By:      /s/ PETER C. NELSON
                                            ------------------------------------
                                                      Peter C. Nelson
                                               President and Chief Executive
                                                           Officer
 
     Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed below by the following persons on behalf
of the registrant in the capacities and on the dates indicated, each of whom
constitutes and appoints Peter C. Nelson or Gerald F. Feeney as the true and
lawful attorney-in-fact of the undersigned, with full power of substitution and
revocation, for and in the name, place and stead of the undersigned, to execute
and deliver the Registration Statement on Form S-3, and any and all amendments
thereto, including without limitation pre-effective and post-effective
amendments thereto; such Form S-3 and each such amendment to be in such form and
to contain such terms and provisions as said attorney or substitute shall deem
necessary or desirable; giving and granting unto said attorney, or to such
person as in any case may be appointed pursuant to the power of substitution
herein given, full power and authority to do and perform any and every act and
thing whatsoever requisite, necessary or, in the opinion of said attorney or
substitute, able to be done in such matter as the undersigned might or could do
if personally present, hereby ratifying and confirming all that said attorney or
such substitute shall lawfully do or cause to be done by virtue hereof.
 
<TABLE>
<CAPTION>
                SIGNATURE                                TITLE                      DATE
- - ------------------------------------------  -------------------------------  -------------------
 
<S>                                         <C>                              <C>
           /s/ PETER C. NELSON               President and Chief Executive     February 17, 1998
- - ------------------------------------------  Officer and Director (Principal
             Peter C. Nelson                      Executive Officer)
           /s/ GERALD F. FEENEY             Vice President, Chief Financial    February 17, 1998
- - ------------------------------------------       Officer and Treasurer
             Gerald F. Feeney                (Principal Financial Officer)
 
           /s/ CALVIN L. BREED              Controller, Assistant Secretary    February 17, 1998
- - ------------------------------------------      and Assistant Treasurer
             Calvin L. Breed                (Principal Accounting Officer)
 
            /s/ ROBERT W. FOY                  Chairman of the Board of        February 17, 1998
- - ------------------------------------------             Directors
              Robert W. Foy
 
     /s/ EDWARD D. HARRIS, JR., M.D.                   Director                February 17, 1998
- - ------------------------------------------
       Edward D. Harris, Jr., M.D.
 
          /s/ ROBERT K. JAEDICKE                       Director                February 17, 1998
- - ------------------------------------------
            Robert K. Jaedicke
 
            /s/ LINDA R. MEIER                         Director                February 17, 1998
- - ------------------------------------------
              Linda R. Meier
</TABLE>
 
                                      II-3
<PAGE>   21
 
<TABLE>
<CAPTION>
                SIGNATURE                                TITLE                      DATE
- - ------------------------------------------  -------------------------------  -------------------
 
<S>                                         <C>                              <C>
 
         /s/ RICHARD P. MAGNUSON                       Director                February 17, 1998
- - ------------------------------------------
           Richard P. Magnuson
 
             /s/ C. H. STUMP                           Director                February 17, 1998
- - ------------------------------------------
               C. H. Stump
 
           /s/ J. W. WEINHARDT                         Director                February 17, 1998
- - ------------------------------------------
             J. W. Weinhardt
</TABLE>
 
                                      II-4
<PAGE>   22
 
                                 EXHIBIT INDEX
 
<TABLE>
<CAPTION>
    EXHIBITS                                DESCRIPTION OF EXHIBITS
    --------     ------------------------------------------------------------------------------
    <S>          <C>
     5.1         Form of opinion of McCutchen, Doyle, Brown & Enersen, LLP.
    23.1         Consent of KPMG Peat Marwick LLP
    23.2         Consent of McCutchen, Doyle, Brown & Enersen, LLP (included in their opinion
                 filed as Exhibit 5).
    24.1         Power of Attorney (included on signature page filed in Part II).
</TABLE>

<PAGE>   1
 
                                                                     EXHIBIT 5.1
 
                     MCCUTCHEN, DOYLE, BROWN & ENERSEN, LLP
 
February 13, 1998                                         Direct: (650) 849-4820
                                                                [email protected]
 
California Water Service Group
1720 North First Street
San Jose, California 95112
 
                       REGISTRATION STATEMENT ON FORM S-3
 
LADIES AND GENTLEMEN:
 
     We have acted as counsel for California Water Service Group, a California
corporation (the "Company"), in connection with the Registration Statement on
Form S-3 as amended (the "Registration Statement") filed by the Company under
the Securities Act of 1933, relating to the registration of shares of Common
Stock, no par value, of the Company.
 
     We are of the opinion that the foregoing securities have been duly
authorized and, when sold pursuant to the terms described in the Registration
Statement, will be duly and validly issued, fully paid and nonassessable.
 
     We hereby consent to the filing of this opinion as Exhibit 5.1 to the
Registration Statement and to the use of our name under the caption "Legal
Matters" in the Registration Statement and in the Prospectus included therein.
 
                                  Very truly yours,
 
                                  McCUTCHEN, DOYLE, BROWN & ENERSEN, LLP
 
                                  By          /s/ WILLIAM J. NEWELL
                                    --------------------------------------------
                                                A Member of the Firm

<PAGE>   1
 
                                                                    EXHIBIT 23.1
 
                        CONSENT OF INDEPENDENT AUDITORS
 
Shareholders and Board of Directors
California Water Service Group:
 
     We consent to the use of our report incorporated herein by reference and to
the reference to our firm under the heading "Experts" in the prospectus.
 
KPMG Peat Marwick LLP
 
San Jose, California
February 13, 1998


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