The transformation of quinoline, isoquinoline and 3-, 4-, 6- and 8-methylquinoline by Desulfobacterium indolicum was compared with that of the N-containing analogues indole and 1-, 2-, 3- and 7-methylindole. The metabolites were identified using high-performance liquid chromatography with UV detection, thin-layer chromatography, combined gas chromatography/mass spectrometry and proton NMR spectroscopy. All degraded compounds were initially hydroxylated at position 2 by D. indolicum. A new degradation product of quinoline was observed in the second transformation step, where 3,4-dihydro-2-quinolinone accumulated. This ring-reduced compound was further transformed into unidentified products. The transformation pathway of indole was characterized by well-known steps through oxindole, isatin, and anthranilic acid. No further transformation of the hydroxylated methyl analogues : 3- and 7-methyloxindole and 3- and 4-methyl-2-quinolinone, was observed within 162 days of incubation. These degradation products accumulated in stoichiometric amounts, while 6- and 8-methyl-2-quinolinone were further degraded to 6- and 8-methyl-3,4-dihydro-2-quinolinone in stoichiometric amounts. Isoquinoline, 2-methylquinoline and 1- and 2-methylindole were not degraded by D. indolicum. These observations indicate that a methyl group at or close to position 2 results in blockage of the microbial attack, and that transformation of hydroxyquinolines methylated at the heterocyclic ring also was blocked or sterically inhibited. An incomplete transformation of some methylated compounds was observed, e.g. for 3- and 6-methylquinoline and 3- and 7-methylindole, with residual concentrations of 0.5-4 mg/l in relation to initial concentrations of 10-15 mg/l.