Deltamethrin is a pyrethroid ester insecticide widely used in agricultural and non agricultural purposes in India, creates a serious threat to the environment as well as target and non target organisms. The present investigation was undertaken to study the effect of lethal (8mg/L) and sublethal (0.8mg/L) deltamethrin level on selected haematological indices of Cirrhinus mrigala. The target fish were exposed to lethal (1, 2, 3 and 4 days) and sublethal (1st, 5th, 10th and 15th day) periods. The results showed that the erythrocytes, Hb, and hematocrit values decreased continuously for both lethal and sublethal concentration of deltamethrin. WBC, MCV and MCH were increased considerably, while the MCHC remained unchanged during the study. In conclusion, the study confirmed the lethal and sublethal effect of deltamethrin on C. mrigala. The decrease in levels of RBCs, Hb, PCV and increase in WBCs revealed the haematological toxicity of this pesticide pollutant and they may be helpful as a tool to monitor the health status of the fish species.