The acute toxicity of ammonia to hatchery-reared rainbow trout Salmo gairdneri was measured in 86 flow-through tests, 96 hours to 35 days long. Fish ranged in age from 1-day-old fry (<0.1 g) to 4-year-old adults (2.6 kg). The 96-hour median lethal concentrations (96-hour LC50) ranged from 0.16 to 1.1 mg/l un-ionized ammonia (11 to 48 mg/l total ammonia-nitrogen). Susceptibility to ammonia decreased as the fish developed from sac fry to juveniles, and increased thereafter. Acute toxicity decreased as temperature increased over the range 12 to 19C. No significant differences in toxicity were observed in tests with different ammonium salts. The LC50 values obtained for 12- and 35-day tests were not appreciably different from those for tests of shorter time periods.