In acute bioassay tests the estuarine clam, Meretrix meretrix when exposed to synthetic pyrethoid Cypermethrin, the behavioural disabilities of clams was different in different concentration. In higher concentration clams showed unhealthy condition. In the present study, it is found that the physico-chemical parameters like temperature, pH and salinity of water also affected the toxicity of the cypermethrin to the estuarine clams.After acute exposure of cypermethrin, the rate of oxygen consumption increased during 12 to 48 hours. This increase was at sublethal and lethal concentration. There was marked increase in LC50 group.Metabolic activity of the clams showed utilization of the biochemical energy to counteract the toxic stress. After acute exposure of Cypermethrin, clams showed remarkable changes in the biochemical composition i.e., total glycogen, protein and lipid content of the various tissue like gill, foot, mantle, and hepatopancreas, male and female gonad.The protein content was decreased in mantle, and foot in LC0 group. In LC50 group protein content was decrease in mantle, foot, male gonad and female gonad.