Early life stages of smallmouth bass (Micropterus dolomieui Lacepede) were exposed to ammonia concentrations at four different pH values (from 6.5 to 8.7). Acute lethal tests were conducted with juveniles, and partial chronic tests were initiated at the late embryo stage and extended through 32 d of exposure. The 96-h LC50 ammonia concentrations at pH 6.53, 7.16, 7.74 and 8.71 were 371, 117, 39.5 and 7.43 mg/l total ammonia-N (NH3-N) and 0.694, 1.00, 1.20 and 1.78 mg/l un-ionized NH3, respectively. The acute toxicity of ammonia in terms of NH3-N was directly related to test pH, and in terms of NH3 was inversely related to pH.