The filamentous fungus Cunninghamella elegans oxidized (+/-) trans-9,10-dihydroxy-9,10-dihydrobenzo[a]-pyrene to a complex mixture of metabolites which were detected by high-pressure liquid chromatography. Two of the metabolites were identified as (+/-)7 beta, 8 alpha, 9 alpha, 10 beta-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene and (+/-)-7 beta,8 alpha,9 beta,10 alpha-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene. A third product gave absorption and mass spectra consistent with a diol-epoxide structure. Hydrolysis of this compound gave (+/-)-7 beta, 8 alpha, 9 beta, 10 alpha-tetrahydroxy-7,8,9,10-tetrahydrobenzo[a]pyrene as the major identifiable product with a minor unidentified tetraol. Synthetic (+/-)-9 alpha, 10 beta-dihydroxy-7 beta, 8 beta-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene gave the same hydrolysis products and had the same retention time on high-pressure liquid chromatography as did the fungal metabolite. The trans-9,10-dihydroxy-9,10-dihydrobenzo[a]pyrene recovered at the end of the experiment showed no optical activity, indicating that both enantiomers were metabolized by the fungus. the results suggest that C. elegans oxidizes (+/-)-trans-9,10-dihydroxy-9,10-dihydrobenzo[a]pyrene to diastereomeric benzo[a]pyrene 9,10-diol 7,8-epoxides.