California Code, Revenue and Taxation Code - RTC § 23362

An affiliated group, subject to the provisions of this article, shall have the privilege of making a consolidated return for the taxable year in lieu of separate returns. The making of a consolidated return shall be upon the condition that all the corporations, which have been members of the affiliated group at any time during the taxable year for which the return is made, consent to all regulations under Section 23363 prescribed prior to the making of such return and the making of a consolidated return shall be considered as such consent. In the case of a corporation, which is a member of the affiliated group for a fractional part of the taxable year, the consolidated return shall include the income of such corporation for such part of the taxable year as it is a member of the affiliated group. In the case of a common parent company which is not itself a common carrier by railroad the consolidated return shall include on a consolidated-return basis only the income received from affiliates which are common carriers by railroad.