Abstract: The 21-day chronic toxicity test of butachlor,pyrazosulfuron-ethyl and bensulfuron-methyl to Daphnia magna was carried out according to "Standard Methods for the Examination of Water and Wastewater" by American Public Health Association and "Toxicity Test Methods" by China.The results showed that when the concentration of these three herbicides increased,both ex-production maturity and reproduction interval decreased,and the number of larva per embryo was up at the beginning but down in the end. The change of rm is quite different,i.e.,from 0.157 2 d-1 to 0.314 0 d-1. While in various concentrations of pyrazosulfuron-ethyl,the change of rm is relatively stable,that is from 0.206 6 d-1 to 0.276 8 d-1. However,the rm of Daphnia magna in bensulfuron-methyl is higher than in other two herbicides i.e.from 0.334 1 d-1 to 0.397 1 d-1. In butachlor solution,the reproduction capacity of Daphnia magna is relatively smaller than in pyrazosulfuron-ethyl and bensulfuron-methyl.