Cal. Rev. & Tax. Code § 18506

A trust that qualifies under Section 401(a) of the Internal Revenue Code and which is exempt is not required to file a return unless it changes the character of its organization, the purpose for which it was organized, or its method of operation or unless the trust has unrelated business taxable income. Where an exempt trust has unrelated business taxable income which exceeds one thousand dollars ($1,000) it shall file a return, verified by an executive officer under penalties of perjury in the form prescribed by the Franchise Tax Board on or before the 15th day of the fourth month following the close of the taxable year.