The metabolism by rat liver 9000 × g supernatant of the environmental carcinogen benzo[b]fluoranthene was investigated. The major metabolites were identified, by comparison to synthetic samples, as 5- and 6-hydroxybenzo[b]-fluoranthene and 4- or 7-hydroxybenzo[b]fluoranthene. The principal dihydrodiol metabolite formed under these conditions was trans-11,12-dihydro-11,12-dihydroxybenzo[b]-fluoranthene, which was identified by comparison to the synthetic compound. 1,2-Dihydro-1,2-dihydroxybenzo[b]fluoranthene was identified, by its mass spectrum and by comparison of its u.v. spectrum to that of a synthetic model compound, 11-ethylidene-11H-benzo[b]fluorene. No evidence was obtained for the formation of 7b,8-dihydro-7b,8-di-hydroxybenzo[b]fluoranthene or trans-9, 10-dihydro-9,10-dihydroxybenzo[b]fluoranthene. The latter would be the precursor to a bay region dihydrodiol epoxide of benzo[b]-fluoranthene.