Abstract: Background and Objective: Indiscriminate use of pesticides may alter the haematological parameters cause’s health hazards in fishes which ultimately affects the physiological condition of the human being. Therefore, the objective of the work was to elucidate the Chlorpyriphos 20% EC induced alteration in haematological parameter to a freshwater fish, Clarias batrachus (Linnaeus, 1758) at its lowest and highest concentrations during 96 hrs static bioassay. Materials and Methods: A 96 hrs full-scale static bioassay test has been performed to the known range of concentrations of Chlorpyriphos 20% EC to calculate LC50 values, presumable safe and safe dischargeable concentrations. Blood samples were collected through the caudal vein and haematological analysis were done. Data were statistically analyzed through software, SPSS (Version 16) and Graph Pad Prism software (version-7). Results: The 96 hrs LC50 values and Confidence Ratio (R) for Chlorpyriphos 20% EC to the Clarias batrachus was estimated to be 217.755 and 1.294 ppb, respectively by Finney's Probit Analysis method. However, safe or harmless and safe dischargeable concentration was also calculated as 77.604 and 1.056 ppb, respectively. The percent alteration (%) in Dissolve Oxygen (DO) content and Free CO2 was observed as 8.108 and 21.621 mg L–1 and 15.116 and 76.744 mg L–1 respectively for Control and 320 ppb concentrated (tested) toxicant solution after 96 hrs. Furthermore, the decreasing trend in RBC (×106 mm3), WBC Count (×103 mm3), PCV (%) and Haemoglobin (g dL–1) were observed whereas MCH (pg/Cell), MCHC (g dL–1) and Glucose Levels (mg dL–1) were found to be increased during an investigation from control to 320 ppb during the course of bioassay. Conclusion: Chlorpyriphos 20% EC are extremely toxic to a freshwater fish, Clarias batrachus (Linnaeus, 1758) which alter the haematological parameters.