The present study was conducted to determine the effects of Methyl parathion on morphological behavioural and some haematological indices in Hypophthalmichthys molitrix (Silver carp). The Silver carp were exposed to three different concentrations of (1, 5 and 10 ppm of the insecticide) for four days. The morphological and behavioural changes were observed and recorded. The values of erythrocytes (RBC- 1.65 ± 0.132), packed cell volume (PCV- 30.55±0.105) and hemoglobin (Hb-11.60±0.056) and haematocrit (Ht-0.64±0.340) value were significantly (P<0.005) decreased as compared to control (4.63±0.240, 48.01 ±0.540, 18.1±0.320 and 2.6±0.46) respectively. The values of mean corpuscular emoglobin content (MCHC-45.76±0.31) and mean corpuscular hemoglobin (MCH 38.30±0.127) were significantly (P<0.05) increased whereas, the mean corpuscular volume (MCV-90.50±0.158) was found to decrease with Methyl parathion administration in comparison with control (34.02±0.33, 31.13±0.076, 94.31±0.489). Lymphocyte (P<0.05), Neutrophils, Eosinophil, and Basophil increased significantly (P<0.005). In conclusion, the changes in blood parameters of the fish exposed to Methyl parathion may be related to decreased immune system. Assessment of haematological parameters could provide a useful indicator of Methyl parathion toxicity in freshwater fish.