The pattern of incorporation of sodium [1,2-13C2]acetate into cardinalin 21by the fruit bodies of the toadstoolDermocybe cardinalisshows that both halves of the molecule are formed biosynthetically from an octaketide in which the C 3 and C 3′ methyl groups come from the acetate starter unit while the C 1 and C 1′ methyls originate from the terminal methylene group. This folding pattern is opposite to that observed during the biosynthesis of pyranonaphthoquinones in bacteria.