The present study was undertaken to evaluate the lethal toxicity and stress levels of commercial formulations of Ammonium Sulphate toward freshwater Mystus vittatus. The 96 h LC50 values, determined in a semi-static system by probit analysis was 500 mg/L for Ammonium Sulphate, indicating that the fish were more sensitive to Ammonium Sulphate. There were large variations in the safe levels estimated by different methods for the metal. In addition to dose and dose-time dependent increase in mortality rate, stress signs in the form of behavioral changes were observed in response to the test salt.