The metabolism of 3-, 8- and 9-fluorobenzo[k]fluoranthene (B[k]F) relative to B[k]F was investigated. The major metabolites of B[k]F formed in vitro using rat liver S-9 metabolism systems were 8,9-dihydro-8,9-dihydroxyB[k]F, the 2,3-quinone of B[k]F and 3-, 8- and 9-hydroxyB[k]F. Fluorine substitution within the structure of B[k]F substantially altered the types of metabolites formed in vitro. The most pronounced effect was observed with 9-fluoroB[k]F. In contrast to B[k]F, the 8,9-dihydro-8,9-dihydroxy-, 9-hydroxy- and 10,11-dihydro-10,11-dihydroxy derivatives were not detected as metabolites of 9-fluoroB[k]F. However, either the 2,3- or 4,5-dihydrodiol of 9-fluoroB[k]F was detected. In the case of 8-fluoroB[k]F, neither the 8- nor 11-hydroxy- derivatives were detected. The principle dihydrodiols formed from 8-fluoroB[k]F were the 10,11-dihydrodiol and either the 2,3-or 4,5-dihydrodiol. The pattern of metabolites formed with 3-fluoroB[k]F was similar to that observed with B[k]F with the exception that neither the 3- nor 4-hydroxy derivatives were formed. Mass spectral data indicated that fluoro substitution is not lost to any appreciable extent during the metabolism of 3-, 8- and 9-fluoroB[k]F. The mutagenic activity of these B[k]F fluoro derivatives along with B[k]F, 2,3-dihydro-2,3-dihydroxyB[k]F, the 2,3-quinone of B[k]F and 8,9-dihydro-8,9-dihydroxyB[k]F were evaluated in Salmonella typhimurium TA100 in the presence of rat liver S-9 metabolism systems. 3-FluoroB[k]F was more mutagenic than B[k]F, while both 8- and 9-fluoroB[k]F were less active. While the 2,3-dihydrodiol and 2,3-quinone were weakly active, the 8,9-dihydrodiol had similar mutagenic potency to B[k]F.