The use of pesticides (such as herbicides, insecticides, and fungicides) increases crop yields and is beneficial in controlling weeds and nuisance organisms, but pesticides can adversely affect the environment (Larson and others, 1997a) and human health. Many pesticides are soluble in water and may enter a surface-water body in a dissolved state. Other pesticides bind to soil particles and can be transported to surface-water bodies through soil erosion. Pesticides bound to soil particles can remain suspended in the water column or can become entrained in the bed sediment. The transport of pesticides from their application areas by water is recognized as a source of contamination, and elevated levels of pesticides in surface water can render the water unfit for human consumption. Pesticides released into the environment can have adverse effects on ecological and human health. Many pesticides are known or suspected carcinogens and can be toxic to humans and aquatic species. Many of the known health effects, however, require exposure to concentrations higher than those typically found in the environment; the health effects of chronic, long-term exposure to low or trace concentrations of pesticides are generally unknown. Other concerns include synergistic effects of multiple pesticides as well as the processes of bioaccumulation, bioconcentration, and biomagnification, which entail the uptake and accumulation of chemical substances by organisms through the food chain: Organophosphorus pesticides (OPs) have been widely used throughout the world since1960. This review discusses the fate of organophosphorus pesticides in the aquatic environment via processes such as adsorption, hydrolysis, oxidation, and photochemical degradation. Further-more, the breakdown products of OPs are discussed, as new research has indicated that the products of degradation can be very harmful as well and because relatively little research has been carried out on comprehensive product identification. Recommended future research areas are highlighted.