Insecticide is common pollutants of freshwater ecosystems where they induce adverse effects on the aquatic biota. Fish, Catla catla is an important carp species in Tamil Nadu region having good nutritional values. Fishes living in close association with may accumulate heavy metals. In the present observation, the toxic effects of the pesticide cypermethrin LC50 0.22 ppm on the total RBC, WBC and Hb in the fish, Catla catla were estimated. Sublethal concentrations of insecticide cypermethrin on (10% and 30% sublethal concentrations) showed a decreasing trend in the RBC and Hb compared to controls and the WBC analysis revealed a significant increased compared to control for a period of 5, 10 and 15 days exposures. The results indicated the toxic nature of the insecticide cypermethrin.