California Water Service GroupCommon Stock Having an Aggregate Offering Price of up to $350,000,000 We have entered into an equity distribution agreement (the “Equity Distribution Agreement”) with Robert W. Baird & Co. Incorporated, BofASecurities, Inc., Morgan Stanley & Co. LLC, Wells Fargo Securities, LLC, Janney Montgomery Scott LLC, RBC Capital Markets, LLC, Blaylock Van,LLC and Samuel A. Ramirez & Company, Inc., each as sales agent and/or, as Forward Seller (as defined below) (in any such capacity, each, a“Manager” and, together, the “Managers”) and the Forward Purchasers (as defined below), providing for the offer and sale of shares of our commonstock, par value $0.01 per share (“our common stock”), having an aggregate gross sales price of up to $350,000,000, from time to time through theManagers, acting as our sales agents, or, if applicable, as Forward Sellers, or directly to one or more of the Managers, acting as principal. Sales of shares of our common stock, if any, as contemplated by this prospectus supplement made through the Managers, as our sales agents or asForward Sellers, will be made by means of ordinary brokers’ transactions on the New York Stock Exchange (the “NYSE”) or otherwise at marketprices prevailing at the time of sale, at prices related to prevailing market prices or at negotiated prices, by privately negotiated transactions (includingblock sales) or by any other methods permitted by applicable law. The Equity Distribution Agreement contemplates that, in addition to the issuance andsale by us of shares of our common stock to or through the Managers, we may enter into separate forward sale agreements (each, a “forward saleagreement” and, collectively, the “forward sale agreements”), with each of Robert W. Baird & Co. Incorporated, Bank of America, N.A., MorganStanley& Co. LLC, Wells Fargo Bank, National Association and Royal Bank of Canada, or one of their respective affiliates (in such capacity, each a“Forward Purchaser” and, collectively, the “Forward Purchasers”). If we enter into a forward sale agreement with any Forward Purchaser, we expectthat such Forward Purchaser or its affiliate will attempt to borrow from third parties and sell, through a Manager, acting as sales agent for such We currently expect to fully physically settle each forward sale agreement, if any, with the relevant Forward Purchaser on one or more datesspecified by us on or prior to the maturity date of such forward sale agreement, in which case we expect to receive aggregate net cash proceeds atsettlement equal to the number of shares specified in such forward sale agreement multiplied by the relevant forward price per share at such time.However, subject to certain exceptions and conditions, we may also elect, in our sole discretion, to cash settle or net share settle all or any portion ofour obligations under any forward sale agreement, in which case we may not receive any proceeds (in the case of cash settlement) or will not receiveany proceeds (in the case of net share settlement), and we may owe cash (in the case of cash settlement) or shares of our common stock (in the case of Sales of our common stock, if any, under this prospectus supplement and the accompanying prospectus, may be made by any method deemed tobe an “at the market” offering as defined in Rule415(a)(4) promulgated under the Securities Act of 1933, as amended (the “Securities Act”). None ofthe Managers, whether acting as our sales agent or, if applicable, as Forward Seller, is required to sell any specific number or dollar amount of sharesof our common stock, but each has agreed, subject to the terms and conditions of the Equity Distribution Agreement, to use its commerciallyreasonable efforts, consistent with its normal trading and sales practices and applicable law and regulations, to sell shares of our common stock on theterms agreed upon by such Manager, us and, in the case of shares offered through such Manager as Forward Seller, the relevant Forward Purchaser We will pay the applicable Manager a commission at a rate equal to 1.0% of the gross sales price of the shares of our common stock sold throughsuch Manager, as our sales agent. In connection with each forward sale agreement, we will pay the applicable Manager, as Forward Seller, acommission, in the form of a reduction to the initial forward price under the related forward sale agreement, at a rate equal to 1.0% of the gross salesprice of the borrowed shares of our common stock sold through such Manager, as Forward Seller, during the applicable forward selling period for suchshares (subject to certain possible adjustments to such gross sales price for daily accruals and any dividends having an “ex-dividend” date during such Under the terms of the Equity Distribution Agreement, we may also sell shares of our common stock to one or more of the Managers as principal,at a price per share to be agreed upon at the time of sale. If we sell shares to one or more of the Manager