Abstract: The toxic effects of three common pesticides (chlorpyrifos,butachlor and triadimefon) to benthos of three native species (Bellamya quadrata,Cipangopaludina cathayensis and Corbicula fluminea) were studied by acute toxicity tests,and the sensitivity of B.quadrata and C.cathayensis in different size to pesticides was measured.The results showed that the 96 h-LC50 values of chlorpyrifos,butachlor and triadimefon to B.quadrata were 4.32,3.62 and 15.2 mg/L,respectively,and those to C.cathayensis were 6.31,4.31 and 16.9 mg/L,respectively,to C.fluminea were 8.75,6.83 and 26.5 mg/L,respectively.Chlorpyrifos and butachlor showed middle toxicity to the three species,and triadimefon showed low toxicity.The young snails of B.quadrata were 2.52,1.84 and 1.72 times more sensitive to chlorpyrifos,butachlor and triadimefon,respectively,than the large snails;and the young snails of C.cathayensis were 2.26,2.26 and 2.67 times more sensitive to the three pesticides than the large snails.The results suggested that we should pay attention to protecting the benthos especially the larvae when applying the three pesticides in the fields.