Carbazole, carbazoles with monomethyl or dimethyls substituted on different positions (C-1-carbazoles or C-2-carbazoles), and benzocarbazoles, as toxic and mutagenic components of petroleum and creosote contamination, were biodegradable by an isolated bacterial strain Pseudomonas sp. XLDN4-9. C-1-carbazoles were degraded in preference to carbazole and C-2-carbazoles. The biodegradation of C-1-carbazoles or C-2-carbazoles was influenced by the positions of methyl substitutions. Among C-1-carbazole isomers, 1-methyl carbazole was the most susceptible. C-2-carbazole isomers with substitutions on the same benzo-nucleus were more susceptible at a concentration of less than 3.4 mu g g(-1) petroleum, especially when harboring one substitution on position 1. In particular, 1,5-dimethyl carbazole was the most recalcitrant dimethyl isomer.