In the present work an attempt was made to assess the acute toxicity of Merit Alpha (an organochlorine pesticide) and its impact on behavioural response of a fresh water air breathing fish, Mystus tengara (Ham.). The LC 50 values for 24, 48, 72 and 96 hrs exposure at 29.0�1.00 for Merit Alpha exposed fishes were - 0.0058, 0.0042, 0.0032 and 0.0020 mg/l respectively. The safe concentration of Merit Alpha was 0.0007 mg/l under the same experimental condition. The mean values of opercular beating decreased gradually with the increasing concentration of Merit Alpha. The behavioural responses of the fish (such as fast opercular beatings for half an hour to one hour, loss of balance, erratic swimming, secretion of copious amount of mucus on the body etc.) of the fish was grossly dependent on concentration and length of exposure.