The toxicity of nitrite to rainbow trout is pH-dependent within the range considered acceptable to most freshwater aquatic life (pH 6.5-9.0). Both of the nitrite species, NO2(-) and HNO2, are toxic. It is recommended that nitrite criteria to protect freshwater aquatic life be based on total nitrite, and that such criteria reflect the pH dependence of nitrite toxicity. Variation in the toxicity of nitrite in the presence of chloride, sulfate, phosphate, and nitrate anions has also been demonstrated. It is concluded that the toxicity of nitrite to fishes, in addition to being pH-dependent, is also dependent in varying degrees upon many of the anions that are commonly found in natural aquatic environments.