Herbicides are important aids in enhancing sport fishing, fish removal, and other recreational uses of lakes and ponds. Simazine, 2-chloro-4,6-bis(ethylamino)-s-triazine, has been tested for several years as an experimental aquatic herbicide and has been shown to be effective in controlling aquatic plants in many different situations (FLANAG~, 1960; SNOW, 1964; SUTTON et al., 1965; WALKER, 1964; and WILE, 1967). The present federal registration proposal states that "ponds should be treated before I0 to 20% of the surface is covered with pond scum (algae mats) and/or while submerged aquatic weeds are actively growing and before they reach the surface of the water." However, the application of herbicides before aquatic vegetation begins growing in the spring, minimizes problems associated with the biological oxygen demand of dying plants. Simazine residues disappear rapidly when aquatic plants are present (SUTTON, 1965), but before this systemic herbicide can be registered for pre-emergent aquatic vegetation use, its residual effects in aquatic ecosystems must be evaluated. Therefore, we monitored and evaluated simazlne residues in the substrate (mud), water, benthic invertebrates, and fish for more than 2 years after the herbicide was applied at four rates in early spring.