Carbaryl was degradaded to monomethylamine(MA) by bacterium. which isolated from lake sediment. Its carbaryl degradation was maximized when grown on mineral salt medium conditioning 20 $mu$M of carbaryl as a sole carbon source at 26${circ}$C and initial pH 7.0-9.0 Its degradation ability was minimized at initial pH 3.0 and 5.0 The percent conversions { (moles of MA in excess of control / mole of carbaryl added) $ imes$ 100} for 10. 20. 40 and 80 $mu$M of carbaryl were 18.6. 16.1. 18.1 and 11.6 respectively. It suggests that increasing amount of carbaryl added above 80 $mu$M. the percent conversion should be decreased. The MA production by the bacterium was lineary related to the cultural time. After 7days culture. its percent conversion was 46.2. and this result suggests that a half amount of carbaryl would be hydrolyzed to MA by the bacterium within 8 days.