An axenic bacterial strain, sp. KB1, was isolated from malathion-contaminated soil. It tolerated malathion up to 0.15 % and, under aerobic conditions, utilized it as sole carbon source. 20 % malathion and 47 % malaoxon were degraded out of the initially provided malathion. Two metabolites, mal-monocarboxylic acid and mal-dicarboxylic acid, were detected within 7 days at 30 A degrees C. Esterase activity of the strain was 240 +/- A 2.5 U/ml after 7 days of growth. Sterilized soil mixed with malathion showed rapid degradation of malathion when inoculated with strain KB1 as compared to the uninoculated soil.