Acute toxicity of Chlorpyriphos 20 % EC to Clarias batrachus (Linnaeus, 1758) has been assessed to understand their toxicity level, 95 per cent confidence limits, Safe or harmless and safe dischargeable concentrations. The experiment was design up to 96 hours and; LC_10, LC₅₀ and LC₉₉ values were calculated for 24, 48, 72 and 96 by Probit analysis statical method. The experimental water of hardness 560 ± 5 mg/l and pH 7.4 ± 0.3 were used during bioassay test. The upper and lower confidence limit was also estimated for LC₅₀ values of 24, 48, 72 and 96 hours. It has been reported that the safe or harmless concentration were too high as compared to safe dischargeable concentration. Behavioral response in the test specimen has been also observed during course of bioassay. Results indicates that Chlorpyriphos 20 % EC were found to be acutely toxic to Clarias batrachus, since their LC_10, LC₅₀ and LC₉₉ values were noticed in ppb.