Pesticides used in agriculture ultimately reach the aquatic environment, get biomagnified and cause severe environmental and health problems. Present investigations have been made to assess the detrimental effects of endosulfan on the blood of exotic freshwater fish, Ctenopharyngodon idella (Cuv. and Val.) after exposure of the fish to two sublethal concentrations of an organochlorine pesticide, endosulfan (0.00075mg/L and 0.001mg/L) for 15, 30 and 45 days. The haematological parameters and calculated erythrocytic indices have been negatively correlated with endosulfan concentration and the experiment length. RBC count, haemoglobin content and haematocrit % values have shown a declining trend. The calculated indices like mean corpuscular haemoglobin (MCH) value and mean corpuscular haemoglobin concentration (MCHC) showed a declining trend while mean corpuscular volume (MCV) showed an increasing trend on all exposure occasions. The variation in erythrocytic indices is a good indicator of polluted aquatic environment.