To obtain basic information for the safe use of pesticides, the aquatic environment of 5 major rivers was surveyed in April and August 1982 for organochlorine and organophosphorus pesticide residues. Regardless of sampling times and sites, pesticide residue levels were very low. Of the organophosphorus pesticides surveyed, only IBP and diazinon were detected in water samples while fenthion, fenitrothion, chlorpyriphos, chlorpyriphos-methyl, chlorfenvinphos, phenthoate, and edifenphos were not detected in any water samples. Residue levels and detection frequencies of IBP or diazinon were much higher in water samples collected in August than those collected in April. Detection frequencies of organochlorine pesticides, on the other hand, were high, but their actual residue levels were very low. BHC was detected nearly in all water samples while aldrin was not found in any samples. Detection frequencies of heptachlor, endosulfan, and o.p`-DDT were considerably high but dieldrin and p.p`-DDT were detected in a few water samples and their residue levels were also very low.