A strain of Mycobacterium flavescens was isolated from polluted sediments. It was capable of utilizing pyrene as a sole source of carbon and energy. When pyrene was supplied as a suspension at 50 micrograms/ml, the generation time was 9.6 h and the rate of pyrene utilization was 0.56 microgram ml-1 day-1. In addition to pyrene, the strain could mineralize phenanthrene (17.7%) and fluoranthene (17.9%), but failed to mineralize naphthalene, chrysene, anthracene, fluorene, acenaphthene and benzo[a]pyrene, as determined by recovery of radiolabeled CO2 in incubations conducted for 2 weeks under growth conditions. Metabolites produced during growth on pyrene were detected and characterized by HPLC and GC-MS. The product of initial ring oxidation, 4,5-dihydroxy-4,5-dihydropyrene was identified, as well as ring-fission products including 4-phenanthroic acid, phthalic acid, and 4,5-phenanthrenedioic acid.