Toxicity effects of pesticide is an important threat to the aquatic environment and its organisms. In the present analysis the acute toxicity of malathion to Labeo rohita were established based on the method of Finney, 1971. Lethal concentration (LC₅₀) forWhich was 9.0μl/L. One tenth of the LC₅₀ value (0.9μl/L) was taken for the sublethal studies according to Sprague (1973). A descending value of lethal concentration (LC₅₀) has been found. The fish showed ungraceful behaviour such as erratic and jerky swimming, attempt to jump out of the water, recurrent surfacing and gulping of air, decreases in opercular movement and copious secretion of mucus all over the body. This surveillance can be considered to monitor the quality of aquatic ecosystems and severity of pollution.