Abstract: A small scale field experiment in a hilly area was carried out to investigate dates and doses of chlorpyrifos sprayed in paddy fields during the rice growing season in that region,and residues of chlorpyrifos in irrigation and drainage waters of the paddy fields were measured at various rice growing periods.It was found that chlorpyrifos residue level was higher in the drainage water than in the irrigating water,and 26.07ug•L^-1.The irrigation water diverted from rivers was also found to have chlorpyrifos residue,with the highest reaching as high as 3.23ug•L^-1,indicating that the river water had already been polluted by chlorpyrifo from its use in paddy fields in the river's upstream area.The acute toxicity ρ of chlorpyrifos was determined to be 1.11mg•L^-1(96 h-LC₅₀)to zebrafish,1.44ug•L^-1(48 h-EC₅₀)to Daphnia magna and 0.193mg•L^-1(72 h-EC₅₀)to algae.According to the acute risk assessment criteria established in the European Union(EU)legislation,chlorpyrifos is determined to be low in risk to fish and algae,but is risky to Daphia magna in the waterbodies around paddy fields.