The toxic effects of Malathion and dimethoate, the two organophosphorous pesticides, on the acetylcholinesterase activity of Samia ricini was studied. 4^th and 5^th instar larvae of eri silkworm were exposed for 72 hours in 0.2 ppm and 0.4 ppm sub-lethal doses of Malathion and dimethoate. Exposure of eri silkworm to the sub lethal doses of both the pesticides caused severe decrease in acetylcholinesterase activity. Highest deviation was observed in the brain 4^th instar larvae exposed to 0.4 ppm sub lethal dose of dimethoate at 72^nd hour.