Two natural ponds were studied to investigate the effects of introduced grass carp on native fishes. Biologically significant deleterious effects were detennined for both fish populations. Largemouth bass populations were reduced in one pond and substantially altered at the second site. Fish population structures shifted and symptoms of overcrowding were evident. Bluegill numbers increased in both ponds. Warmouth were greatly reduced in one pond. Several species offorage fishes were eliminated.