Triploid grass carp (Ctenopharyngodon idella) were stocked at a mean length of 229 mm (total length) into two small South Dakota lakes in 1985.Charasp. was the predominant aquatic macrophyte in both lakes. Prior Lake contained a fish community in which the largemouth bass (Micropterus salmoides) was the only top-level predator. An introduction of 49 grass carp per hectare resulted in nearly complete elimination of vegetation within two years. Northern pike (Esox lucius) was the dominant predator in East Lake Eureka. Grass carp were introduced at a density of 61 per hectare, but no impact on aquatic vegetation was noted. We believe northern pike predation reduced grass carp numbers. Therefore, we suggest that further work is needed on size of grass carp stocked in waters containing northern pike.