The biosynthesis of integric acid, a secondary metabolite of the wood-decay fungus Xylaria feejeensis strain 2FB-PPM08M, has been studied. Labeling experiments using [1-(13)C], [2-(13)C] and [1,2-(13)C2] acetate and L-methionine (methyl-(13)C) were separately performed with fungal culture. The labeling patterns of these metabolites indicated the same origin, and determined that integric acid was formed through the condensation of a sesquiterpene and a polyketide. These experiments showed that side chain of compounds would be synthesized by the polyketide pathway, while the ring carbon indicated the biosynthesis of compounds via the mevalonate pathway.