A potential bacterial culture (PI), isolated from garden soil and identified as Pseudomonas pseudoalcaligenes-KPN, was used as a starter seed to develop the biomass in a completely mixed activated sludge (CMAS) reactor and the system was evaluated for treatment of wastewater containing pyridine. The results of this study indicate that pyridine could be degraded efficiently at a loading of 0.251 kg pyridine kg MLSS-1 d(-1) (0.156 kg TOC kg MLSS-1 d(-1)) and at an optimal hydraulic retention time (HRT) of 24 h. Pyridine was used as the sole source of carbon and nitrogen by the biomass. Ammonia-nitrogen (NH3-N) was formed due to the metabolism of the pyridine ring. In the present investigation, the performance of CMAS with reference to pyridine biodegradation and the bio-kinetic constants for the biodegradation of pyridine, in a continuous system, were computed. The results indicate that a CMAS system inoculated with P. pseudoalcaligenes-KPN, under optimum conditions of HRT and pyridine loading, gives a yield coefficient of ( Y) 0.29, decay coefficient (K-d) 0.0011 d(-1), maximum growth rate constant (mu(max)) 0.108 d(-1) and saturation rate constant (K-s) 5.37 mg L-1 for pyridine. (c) 2005 Published by Elsevier Ltd.