Abstract: The acute toxicity of cadmium chloride to grass carp was investigated by stagnophile testing, as well as the effect of superoxide dismutase systems (SOD) on grass carp's liver, kidney and gill tissues. The results showed that the LC50 for grass carp are 45.58, 34.81, 28.63 and 24.05 mg·L^-1 when the acting time of acute toxicity of cadmium chloride are 24, 48, 72 and 96 h, respectively. The high ecological toxicity of cadmium is thus proved to grass carp. The activity of SOD in grass carp's liver and kidney tissues decreases first then increases and decreases once again when Cd²⁺ presents low concentration pollution, while the activity of SOD is inhibited obvious all along (p<0.05) when Cd²⁺ presents high concentration pollution. However, the activity of SOD in grass carp's gill tissues decreases first then increases when Cd²⁺ presents low concentration pollution, while the activity decreases first then increases and then decreases once again when Cd²⁺ presents high concentration pollution. It was revealed that the activity of SOD in grass carp's gill tissues is much lower than in liver and kidney tissues.