respectively and median lethal concentration (LC50) of lead to Caspian sea kutum (R. frisii kutum)for 24, 48, 72 and 96 h of exposure as 315.841, 298.456, 281.419 and 268.065 ppm, respectively. LC50 increased with decrease in mean exposure times for both metals. Physiological responses like rapid opercular movement and frequent gulping of air was observed during the initial stages of exposure after which it became occasional. All these observations can be considered to monitor the quality of aquatic ecosystem and severity of pollution. Hence, concluded that copper is more toxic than lead for Caspian sea kutum