Achromobacter xylosoxidans CS5, capable of utilizing endosulfan as the sole carbon, sulfur and energy source, was isolated from the activated sludge. Degradation of endosulfan by strain CS5 was examined by HPLC. Analysis of culture pH, cells growth, and residual endosulfan demonstrated that CS5 could degrade more than 24.8 mg/l alpha-endosulfan and 10.5mg/l beta-endosulfan after 8 days in aqueous medium, with the formation of endosulfan diol and endosulfan ether as the major metabolites. Cell-free extract of strain CS5 was able to metabolize endosulfan rapidly, and the degradative enzymes were constitutively expressed. Inoculation of strain CS5 was found to promote the removal of endosulfan in soil. Our results suggested that A. xylosoxidans CS5 might degrade endosulfan by a non-oxidative pathway. In addition, detoxification of endosulfan was evaluated using a Salmonella typhimurium TA1535/pSK1002 (umu-test). These finding suggested that the metabolism of endosulfan by strain CS5 was accompanied by significant reduction in the toxicity.