Flow-through bioassays on the acute toxicity of nitrite to rainbow trout (Salmo gairdneri) of four different sizes (2–235 g) showed median lethal concentration (LC50) values for 4 days ranging from 0.19 to 0.39 mg/liter NO2–N. For 12-g rainbow trout the asymptotic LC50 was 0.14–0.15 mg/liter NO2–N after 8 days.