Cal. Bus. & Prof. Code § 26226.7

If a vacancy on the board of directors occurs, except by expiration of term, the remaining members of the board, by a majority vote, shall fill the vacancy, unless the bylaws provide for an election of directors by districts. If the bylaws provide for an election of directors by districts, the vacancy shall be filled either by the election of a director from the district in which the vacancy occurs or by the board of directors calling a special meeting of the members or stockholders in that district to fill the vacancy.