The present study gives a detailed account of physico-chemical parameters, Cypermethrin induced toxicity to estuarine clams K. opima and its effects on behavior and oxygen consumption of clams in each season of Bhatye estuary, Ratnagiri, (Maharashtra). It also deals with Cypermethrin induced biochemical and cellular changes in soft tissues of estuarine clams after acute and chronic exposure in each season. While conducting seasonal bioassay tests, different water parameters have been studied. It is found that the physico-chemical water parameters of the estuarine water affected the toxicity of Cypermethrin to the clam K . opima. Toxicity experiments conducted during three different seasons gave different values of LC0 and LC50. When estuarine clams were exposed to Cypermethrin during three different seasons, it was observed that clams were more sensitive to Cypermethrin in summer and comparatively less during monsoon and winter. This probably relates to high water temperature in summer and low oxygen content of the estuarine water in this season. In monsoon and winter, plenty of food availability and favourable physiological conditions increased the resistance of animal to Cyperme