The effect of widely used commercial-grade organophosphorus insecticides, namely, fenitrothion and ethion, on acetylcholinesterase activity and acetylcholine levels in brain, fat body, and silk gland of Bombyx mori on Days 1, 3, and 5 and food intake of the fifth instar were studied. Acetylcholinesterase activity was found to be inhibited, followed by a concomitant increase in acetylcholine levels in the target tissues. These changes suggest an acceleration in energy metabolism of nerve cells, leading to death of the insects. Food consumption during the fifth instar underwent a significant decrease at lethal doses and an insignificant increase at sublethal doses of fenitrothion and ethion. The overall changes observed in acetylcholinestease, acetylcholine, and food intake due to fenitrothion and ethion were, insecticidewise, in the order fenitrothion