The grass carp (Ctenopharyngodon idella), at a density of 50 1- to 2-kg fish per hectare, satisfactorily controlled filamentous algae in fish hatchery ponds. It was not predacious on several abundant forage species, including fingerling channel catfish (Ictalurus punctatus), fingerling F1hybrids of the bluegill (Lepomis macrochirus) × green sunfish (L. cyanellus) and redear (L. microlophus) × green sunfish, crayfish (Procambarus sp.), bullfrog tadpoles (Rand catesbeiana), fathead minnows (Pimephales promelas), green sunfish, and mosquitofish (Gambusia affinis).