The present study indicates the toxic effects of Copper Sulphate on fish. The sublethal toxicity of Copper Sulphate on protein content in liver, kidney and muscle of fish Mystus vittatus have been studied. The fish exposed to sublethal concentration of 1/20th (low), and 1/10th (high) for the period of 10, 20 and 30 days. The results showed decrease in the protein content and increase in the amino acid content in liver, kidney and muscle of fish Mystus vittatus. The significant reduction in protein content and elevated levels of amino acids is apparently indicative of the toxic effect of Copper Sulphate on biochemical parameter and organism’s response to the toxic stress.