Crayfish have long been a nuisance in fish-rearing ponds at fish hatcheries. The rusty crayfish (Orconectes rusticus ) has displaced endemic species and caused serious declines of aquatic plants in some ponds and lakes in the midwestern USA. The authors attempted to evaluate the effect of intensive trapping on a crayfish population and to identify a selective chemical control agent and evaluate its effectiveness under field conditions. A crayfish population in a small pond was suppressed but not eliminated by trapping: adults were effectively harvested but efficiency diminished sharply as the population declined. Of 19 chemicals tested as possible control agents for crayfish, a synthetic pyrethroid (Baythroid) was by far the most toxic; 25 mu g/L produced a complete kill of crayfish in the pond and was also the most selective for crayfish in laboratory tests.