Food consumption and growth rates of a predator, the largemouth bass (Micropterus salmoides), and the behavior of both the predator and its prey, the mosquitofish (Gambusia affinis), were sensitive indices of sublethal effects of ammonia on predator-prey interactions. Prey consumption and growth rates of bass in control tests increased with increasing prey densities. Ammonia concentrations of 0.63 and 0.86 mg/liter NH3substantially decreased prey consumption and growth of bass in test tanks stocked with densities of 30, 60, and 120 mosquitofish. At a given ammonia concentration, there were greater decreases in prey consumption and growth of bass at higher prey densities. This can be attributed in part to the bass being more sensitive than mosquitofish to ammonia and to the harassment of the predator by the prey which occurred at high ammonia concentrations and high prey densities.