Biosynthesis of ochratoxin A by Aspergillus ochraceus Wilh. was investigated by radiolabeling experiments in which phenylalanine-1-14C and sodium acetate-2-14C were supplied to the fungus in sucrose-yeast extract medium. Results showed that phenylalanine was incorporated unaltered into the phenylalanine moiety of ochratoxin A, whereas the isocoumarin moiety of ochratoxin A was mostly derived via acetate condensation.