A secondary metabolite was isolated from cultures of a Drechslera avenae pathotype with host specificity to Avena sterilis and identified as the macrodiolide (8R,16R)-(-)-pyrenophorin (8,16-dimethyl-1,9-dioxa-cyclohexadeca-3,11-diene-2,5,10,13-tetraone). A considerable yield of the substance was