Cal. Corp. Code § 13277

When a vacancy on the board of directors occurs other than by expiration of term, the remaining members of the board, by a majority vote, shall fill the vacancy, provided that when the by-laws provide for an election of directors by districts, the vacancy shall be filled by the election of a director from the district in which the vacancy occurs; or the board of directors may call a special meeting of the members in that district to fill the vacancy.