The acute toxicity (96-hr LC50) of un-ionized ammonia to mysids (Mysidopsis bahia) and larval inland silver-sides (Menidia beryllina) was influenced by pH and salinity in a species specific manner. With mysids, NH3 was most toxic at pH 7.0 and less toxic at pH 8.0 and 9.0. In contrast, NH3 toxicity to inland silversides was greatest at pH 7.0 and 9.0 and lowest at pH 8.0. A drop in salinity from 31 g/kg to 11 g/kg uniformly increased toxicity to mysids over this pH range. In contrast, in silversides at 11 g/kg, NH3 toxicity was less at pH 7.0, greater at pH 8, and slightly less at pH 9, relative to the toxicity at 31 g/kg. Temperature had no significant effect on the acute toxicity of un-ionized ammonia with acclimated mysids tested at 18, 25 and 32.5C, but did have a small effect with acclimated larval sheepshead minnows (Cyprinodon variegatus) tested at 13, 25 and 32.5C. The chronic toxicity value (the geometric mean of the highest no-effect concentration and lowest effect concentration) at pH 8.0, 25C and 31 g/kg salinity is 0.061 mg NH3/L for inland silversides and 0.232 mg NH3/L for mysids; the acute:chronic ratio is 21.3 and 7.2, respectively.