The environmental fate of the organophosphate insecticide phorate and its metabolites, phorate sulfone and phorate sulfoxide was examined in mesocosms placed in South Dakota wetlands. Three treatment concentrations of phorate (1.2, 2.4, and 4.8 kg/ha) were applied to wetland mesocosms. Phorate, phorate sulfone, and phorate sulfoxide were present in water and sediment during the entire study (28 days) in all treatments. Maximum phorate concentrations in water were measured one day after treatment and decreased significantly during a 28-day period. The metabolites of phorate increased significantly in the water during the study. Phorate was present in higher concentrations in sediments than either phorate sulfone or phorate sulfoxide throughout the study.