Trimethylamine, dimethylamine and methylamine (actually existing as a salt form in the culture medium) were identified as the intermediates of alkyltrimethylammonium salts in activated sludge obtained from a municipal sewage treatment plant. It was considered that the quaternary ammonium salts with long alkyl chains were degraded to tertiary amine by N-dealkylation at the first stage of the biodegradation pathway. The tertiary amine formed in this pathway rapidly disappeared. In the activated sludge, biodegradabilities based on biochemical oxygen consumption and dissolved organic matter were 7.2-53.7% and 97.4-100%, respectively. These results and the disappearance of intermediates as described above indicate that long chain alkyltrimethylammonium salts are ultimately biodegradable.