Halosulfuron-methyl, is a sulfonylurea-based post emergence herbicide. It has gained wider application in agricultural management especially for Rice and Sugar cane, due to its high efficacy activity at low doses. In spite of its agricultural advantage, unintended release of herbicides into different water bodies by agricultural runoffs would definitely harm aquatic species, especially fish, which are primary serves as bioindicators of environmental health. The present study is conducted to evaluate acute toxicity of Halosulfuron-methyl herbicide on freshwater fish Cyprinus carpio L., additionally to assess behavioral responses in exposure conditions. Laboratory assays are performed using 10 fish per group in test chemical, which are under static conditions, for about 96 hour of exposure period. The median lethal concentration (LC₅₀) has been determined in addition to mortality rate. Abnormal behavioral responses are also observed, depending on dose response manner. Lateral swimming, loss of equilibrium with sinking, repeated schooling behavior, fright response, upward and backward swimming, dashing and burst movements, whirling and corkscrew motion, excessive opercular activity and rapid fin beating were observed in behavioral studies. Mucus is secreted in this period. These responses are due to suggestive neuro muscular distress, respiratory dysfunction and physiological stress targeted by herbicide.