Heavy metal pesticides are common pollutants of freshwater ecosystems where they induce adverse effects on the aquatic biota. Estuarine mud crab, Scylla serrata is an important crustacean species of Agniyar estuary in Thanjavur region having good nutritional values. Crabs living in close association with the sediment may accumulate copper sulphate. In the present investication, the toxic effects of the copper sulphate LC50 4.85 ppm (CuSO4) on some biochemical characteristics (total protein in muscle, gill and hepatopancreas) of the estuarine mud crab, Scylla serrata were estimated. There is decreased in all tissues on comparison with control. The results indicated the toxic nature of the heavy metal copper sulphate.