The freshwater fish, Channa gachua was exposed to sub-lethal concentrations of an organophosphorus pesticide, monocrotophos. The fish was exposed to acute toxicity of monocrotophos for 16, 24 and 48 hours while for chronic toxicity it was exposed for 15, 30 and 45 days. The monocrotophos was found to be toxic to the fish, which was found to be increasing with the increase of its concentration. Alterations in various biochemical parameters of fish were observed. The exposure of 0.072 ppm concentration of monocrotophos led to the addition in cholesterol, alkaline phosphatase in plasma, triglyceride in plasma, serum bilirubin, serum creatinine, SGPT and SGOT while reduction in plasma glucose, total protein and lipid peroxidation.