The enantioselective toxicity of the chiral herbicides Rac- and S-metolachlor to Scenedesmus obliquus and Chlorella vulgaris was determined, and the effect of humic acid was studied by using acute toxicity testing method. The results indicated that the toxicity of Rac- and S-metolachlor increased with increasing concentration and exposure time. The EC(50, 96 h) ratio of Rac-metolachlor to S-metolachlor was 2.25 for C. vulgaris and 1.81 for S. obliquus, indicating that S-metolachlor had higher effect on two algaes, and S. obliquus was more sensitive to Rac- and S-metolachlor. Linear correlation between toxicity on S. obliquus and C. vulgaris was observed. The toxicity of Rac- and S-metolachlor changed with humic acid, with more significant change was observed in S-metolachlor (P<0.05).