Pseudomonas sp. strain 7-6, isolated from active sludge obtained from a wastewater facility, utilized a quaternary ammonium surfactant, n-dodecyltrimethylammonium chloride (DTAC), as its sole carbon, nitrogen, and energy source. When initially grown in the presence of 10 mM DTAC medium, the isolate was unable to degrade DTAC. The strain was cultivated in gradually increasing concentrations of the surfactant until continuous exposure led to high tolerance and biodegradation of the compound. Based on the identification of five metabolites by gas chromatography-mass spectrometry analysis, two possible pathways for DTAC metabolism were proposed. In pathway 1, DTAC is converted to lauric acid via n-dodecanal with the release of trimethylamine; in pathway 2, DTAC is converted to lauric acid via n-dodecyldimethylamine and then n-dodecanal with the release of dimethylamine. Among the identified metabolites, the strain precultivated on DTAC medium could utilize n-dodecanal and lauric acid as sole carbon sources and trimethylamine and dimethylamine as sole nitrogen sources, but it could not efficiently utilize n-dodecyldimethylamine. These results indicated pathway 1 is the main pathway for the degradation of DTAC.Quaternary ammonium compounds (QACs) containing a long-chain alkyl group or a benzyl group are cationic surfactants that are widely used in several applications, including as antistatic agents, emulsifiers-dispersants, dye auxiliaries, surface treatment agents, osmotic agents, and hair rinses (8). QACs are also contained in synthetic detergents to reduce static electricity in clothing and improve fabric suppleness. In addition, the bactericidal and fungicidal properties of these compounds, as well as their ability to damage cell membranes and to denature cell proteins, have favored their widespread use in domestic cleaning products (1, 6, 7). Since most of the above-mentioned products are released into the environment through routine disposal wastewater, the accumulation and aquatic toxicity of quaternary-ammonium-based surfactants have been the focus of several studies (4,20).Several researchers have reported adaptation to QACs by aquatic organisms through their repeated exposure to these compounds (19) and the biodegradation of QACs by pure cultures of bacteria (18). McBain et al. showed that repeated exposure of pure cultures, especially Ralstonia sp., altered their susceptibility to QACs (11). In addition, a mixture of Pseudomonas sp. and Xanthomonas sp. isolated from soil and sewage grew well on medium containing decyltrimethylammonium salt as the sole carbon source. Xanthomonas sp. oxidized the terminal carbon of the alkyl chain of QAC (2). Pseudomonas sp. strain B1, isolated from activated sludge, grew well on hexadecyltrimethylammonium chloride (the C16 alkyl QAC in this report), using the compound as a carbon and energy source (17). However, strain B1 could not utilize the intermediate, trimethylamine, as a nitrogen source. Although these findings indicate the metabolic fates of QACs in an aquatic envir...