Scylla serrata is an important crustacean species in Agniyar estuary region having good nutritional value. The present study reflects the effect of copper sulphate on Lactate Dehydrogenase (LDH) enzyme in some vital organs of the estuarine mud crab Scylla serrata. Crabs of equal size were treated with different concentrations of copper sulphate (0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 ppm) respectively. The mortality rate was noted up to 96 hours. After deducting the LC50, the crabs were treated with a sub-lethal concentration of copper sulphate (2.0 ppm) for 5, 10 and 15 days respectively. The lactate dehydrogenase (LDH) enzyme activity estimation was done in the case of sub-lethal concentrations (10 % and 30 %) of copper sulphate exposure and compared with the control group of crabs. There is increased in all tissues on comparison with control. The results indicated the toxic nature of the heavy metal copper sulphate.