Abstract: The influence of PH and temperature on the toxicity of cypermethrin has been evaluated by using freshwater fishes. Both PH and temperature were found to affect significantly the toxicity of pesticide by altering the physiological responses of fishes resulting in mortality. The adverse effects of the PH and temperature were decreased oxygen consumption, acceleration of chemical reaction, impaired respiratory activities, insufficient energy liberation and so on. In general these environmental factors could lead to a higher level of bioaccumulation of the toxic chemicals, thus causing the chemical stress, which affecting the normal functioning of the body.