Opaliferin, a polyketide with a unique partial structure in which a cyclopentanone and tetrahydrofuran were connected with an external double bond, was isolated from the insect pathogenic fungus Cordyceps sp. NBRC 106954. The structure and relative configuration of opaliferin were determined by spectroscopic analysis and X-ray crystallography. The absolute configuration was established by anomalous dispersion effects in X-ray diffraction measurements on the crystal of di(p-bromobenzoyl) ester of opaliferin. A plausible biosynthetic pathway for opaliferin is proposed.