Studies in large and small ponds conducted on the Auburn University Fisheries Research Station from 1981 through 1983 suggest that gizzard shad (Dorosoma cepedianum) severely competed with largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus) populations. Gizzard shad introductions lowered bluegill reproductive success and caused an imbalance the first year of introduction. Gizzard shad grew rapidly through the size range where they were vulnerable to largemouth bass predation; in this 3-year study, largemouth bass were unable to control gizzard shad numbers. Within 2 years after introduction, gizzard shad comprised half the standing stock while replacing portions of the carrying capacity normally occupied by largemouth bass and bluegill. A proposed model of competition is that gizzard shad populations, even at low levels, filtered out enough food items to reduce survival of small bluegill. The process accelerated as gizzard shad populations expanded; with less bluegill reproduction available as prey, largemouth bass failed to recruit.