A series of monomethyl and dimethyl derivatives of benzofluoranthene (BbF) and benzo[k]fluoranthene (BkF) were synthesized in order to investigate the environmental occurrence and structural requirements for carcinogenicity of methylated nonalternant polycyclic aromatic hydrocarbons. 9-Methyl-BbF (5), 12-methyl-BbF (6), and 1-methyl-BbF (7) were prepared from the appropriate oxotetrahydro-BbF's (17–19). 8-Methyl-BbF (8) was synthesized from 1-methyl-3-oxo-1,2,3,10b-tetrahydrofluoranthene (20) in 11 steps. 3-Methyl-BbF (9) and 1,3-dimethyl-BbF (10) were prepared from 3-methyl-1-oxo-1,2,3,3a-tetrahydrobenzo[b]fluoranthene (31), which was synthesized from methyl 11H-benzo[b]fluorene-11-carboxylate (28). 7-Methyl-BbF (11) was obtained by condensation of 1-methylfluorene (33) with o-bromobenzaldehyde, followed by treatment with KOH and quinoline. 5,6-Dimethyl-BbF (12) was synthesized by reaction of 2,3-dimethylbutadiene with acephenanthrylene (38) followed by aromatization. 8-Methyl-BkF (13) was synthesized from 8-oxo-8,9,10,ll-tetrahydro-BkF. 9-Methyl-BkF (14) was prepared by Friedel-Crafts reaction of 2-methylsuccinic anhydride with fluoranthene, followed by Wolff-Kishner reduction, cyclization, LiAlH4reduction, dehydration, and aromatization. 2-Methyl-BkF (16) was synthesized by an analogous sequence, beginning with 2-methylfluoranthene and succinic anhydride. 7,12-Dimethyl-BkF (15) was prepared by a two-step reduction of 7,12-dicyano-BkF (51).