Two groups of 20 yearling rainbow trout were exposed continuously to inflowing water containing 0.15 and 0.55 mg/liter nitrite as nitrite nitrogen (NO2N), for 48 and 24 hr, respectively. Both groups developed methemoglobinemia but mortality occurred only at the highest concentration (55%). Salmon exposed to 0.50 mg/liter NO2N for 24 hr developed methemoglobinemia and suffered a 40% mortality. Fingerling trout were less sensitive to nitrite than yearlings.