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-C-13], [2-C-13] and [1,2-C-13(2)] acetate and l-methionine (methyl-C-13) 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.