The single and combined effects of Cd2+ and S-metolachlor on acute toxicity, total soluble protein content, superoxide dismutase (SOD) activity and cell membrane permeability of Scenedesmus obliquus (S. obliquus) were studied using the standard toxic testing methods. The results showed that the EC50 of Cd2+ and S-metolachlor decreased with time, and the acute toxicity of S-metolachlor was higher than that of Cd2+, EC(50-24h) of Cd2+ and S-metolachlor was 0.27 mg x L(-1) and 0.24 mg x L(-1), respectively, and EC(50.96h) was 0.16 mg x L(-1) and 0.13 mg x L(-1), respectively. The combined toxicity of Cd2+ and S-metolachlor showed a synergistic effects at low concentration, and antagonism effects at high concentration. After 96h-exposure, the total soluble protein content of S. obliquus decreased, the SOD activity first increased and then decreased, and the cell membrane permeability increased with the increasing concentration of both single and combined treatment.