AbstractPhytoalexin produced by aspen in response to infection byHypoxylon mammatumwas isolated and found to be active against ascospore germination ofHypoxylon mammatumand conidial germination ofAlternariasp. Analysis carried out by thin layer chromatography suggests that the active compounds are phenolic glycosides. These compounds gave an inhibition up to 92.5% when tested against the germinationof Alternariaspores.