The purpose of the study was to explore microbial transformations in cultures of basidiomycetes Pleurotus ostreatus, two bicyclic unsaturated lactones occurring in the form of two diastereoisomers. Some of these strains were able to transform unsaturated lactones into four known and three new derivatives. The structures of all lactones were established on the basis of spectroscopic data. Both substrates and products caused a complete inhibition of growth of A. alternata and F. linii strains.