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 microg g(-1) petroleum, especially when harboring one substitution on position 1. In particular, 1,5-dimethyl carbazole was the most recalcitrant dimethyl isomer.