The toxic effects may include both lethal and sublethal concentrations, which may change the growth rate, development, reproduction, histopathology, biochemistry, physiology and behavior on target organisms and undesirable perturbations in the environment. The result of the present work i.e., observed percentage mortality of species of fresh water fish Cirrhinus mrigala when exposed to the 10% emulsifiable concentration (E.C.) cypermethrin for time periods 24, 48, 72 and 96 h in static system toxicity is in the range of 2.69 ppb, 2.61 ppb, 2.41 ppb, 2.28 ppb Respectively. In the present investigatiion, the test species C. mrigala has shown differential toxicity level with a functin of period. With the increase period of exposure (96 h), the fish showed mortality at less concentration and with decrease of duration of exposrue the fish exhibited mortality at higher concentration. In the present investigation, when fish were exposed to sublethal concentration of cypermethrin for 4 days. Several behavioural changes were observed which include loss of schooling behaviour, swimming near the water surface, hyper activity, erratic movements, seizures, loss of buoyancy, elevated cough, restlessness before death, darting movements and hitting against the walls of test tanks were noticed in all the species tested. A film of mucus was also observed all over the body and also on the gills.