The feeding activity of grass carp (Ctenopharyngodon idella) significantly reduced the final biomass of submerged plants in Missouri ponds. Water quality was improved; total alkalinity and pH were sustained at levels more suitable for productivity. Nocturnal oxygen depletion was less severe. and noxious phytoplankton blooms did not develop in ponds stocked with grass carp. Grass carp had no significant effect on abundance of glass shrimp (Palaemonetes kadiakensis); bluegills (Lepomis macrochirus) had a much greater negative effect. In six of eight ponds with grass carp the densities of fathead minnows (Pimephales promelasl were greater than in any control pond. Young bluegills were significantly more abundant in ponds with grass carp. Bluegill growth was negatively correlated with bluegill density, but not with grass carp biomass. Average total apparent fish production was 270% greater in ponds with grass carp than in control ponds.