Abstract: Metabolic activation of chlorpyrifos to chlorpyrifos-oxon via cytochrome-P450 (CYP450) in the riceland prawn Macrobrachium lanchesteri was examined. Direct effects of chlorpyrifos-oxon on Acetylcholinesterase (AChE) activity was studied in vitro by exposing M. lanchesteri supernatants to chlorpyrifos-oxon. And the role of CYP450 in the bioactivation was investigated in vivo by treating M. lanchesteri with piperonyl butoxide, a potent CYP450 inhibitor, prior to chlorpyrifos exposure. The study indicated that chlorpyrifos-oxon was a more potent AChE inhibitor than chlorpyrifos. However, chlorpyrifos concentrations which did not inhibit AChE activity in vitro exerted AChE inhibition in vivo. CYP450 played an important role in the bioactivation of chlorpyrifos since M. lanchesteri pretreated with 500 μg L-1 piperonyl butoxide for 24 h prior to 1.5 μg L-1 chlorpyrifos exposure for 96 h significantly attenuated the inhibition of AChE activity caused by chlorpyrifos. Nevertheless, pretreatment with piperonyl butoxide did not alter the effects of chlorpyrifos in reducing catalase activity, increasing lipid peroxidation as evidenced by increased thiobarbituric acid reactive substance levels and inducing histopathological changes such as swelling of the gills and degeneration of the hepatopancreatic cells. In conclusion, chlorpyrifos was bioactivated to chlorpyrifos-oxon in M. lanchesteri via CYP450. Chlorpyrifos-oxon was a strong AChE inhibitor but did not alter the effects of chlorpyrifos in inducing oxidative stress and histopathological changes.