rease slightly during theearly stages of biodegradation and then sharply decrease after level 4, when preferential depletion of alkylated carbazolescompared to benzocarbazoles and dibenzocarbazoles or naphthocarbazoles is observed. The susceptibility of alkylated carbazoleisomers to biodegradation decreases with increasing carbon number of the alkyl substituents. Furthermore, nitrogen(N–H) shielded or partially shielded compounds substituted in the 1 and/or 8 positions seem more susceptible to biodegradationthan their nitrogen-exposed counterparts. For example, biodegradation resulted in the preferential removal of 1-methylcarbazole relative to the other methylcarbazole isomers and among the dimethylcarbazoles; 1,8-dimethylcarbazoleseems more susceptible to biodegradation than other isomers, even though some exceptions do occur in our sample set.The benzocarbazole ratio [Nature 383 (1996) 593] decreased with increasing degree of biodegradation. Benzo[b]carbazolehas the highest apparent ability to resist biodegradation among the benzocarbazole isomers.