Cal. Penal Code § 5008

The Secretary of the Department of Corrections and Rehabilitation shall deposit any funds of inmates in his or her possession in trust with the Treasurer pursuant to Section 16305.3 of the Government Code. However, the Secretary of the Department of Corrections and Rehabilitation, shall deposit those funds of inmates in interest-bearing bank accounts or invest or reinvest the funds in any of the securities that are described in Article 1 (commencing with Section 16430) of Chapter 3 of Part 2 of Division 4 of Title 2 of the Government Code and for the purposes of deposit or investment only may mingle the funds of any inmate with the funds of other inmates. Any interest or increment accruing on those funds, less expenses incurred in the investment, shall be deposited in individual inmate or parolee trust accounts on a proportional basis depending upon the amount of funds each individual inmate or parolee account has on deposit.