Cypermethrin is a synthetic pyrethoid mainly used against mango insect pests. Clams are good indicators of pesticide pollution and are known to be tolerant to pesticide accumulation. Acute toxicity experiment of Cypermethrin is carried out on estuarine clam, Marcia opima. The clam exhibited significant increase in protein content of gill in LC0 and LC50 group of clams while foot, male gonad and female gonad exhibited considerable decrease in protein content. Whereas, in LC0, mantle showed high protein content. In LC50 group, hepatopancreas exhibited high protein content.