An attempt was made to detect pesticides remaining in river water and raw water for tap water supply, located in an agricultural area. As a result, 11 kinds of pesticides were detected. The residual pesticides in water were also degraded to by-products, since chlorine is a strong oxidant that does not exist in natural conditions. Organophosphate pesticides containing P = S bonds were easily degraded and produced oxons (P = O) as a primary by-product. Thiobencarb was also degraded by chlorination, producing chlorobenzyl alcohol, chlorotoluene, chlorobenzyl chloride, chlorobenzoic acid and chlorobenzyl aldehyde as chlorination by-products. Also, when thiobencarb was detected in raw water, the above by-products were detected in the filtered water in a purification plant for water supply. Therefore, the management and control of pesticides in drinking water and ambient water quality management should include testing for chlorination by-products.