Coho (Oncorhynchus kisutch) and sockeye (O. nerka) salmon smolts and steelhead trout (Salmo gairdneri) smolts were exposed to several concentrations of air-supersaturated water in the laboratory from March through June, 1974, the normal fish migration period in the Columbia River and the time when the river water is usually supersaturated. Tests were conducted at 12C in water 60 cm deep. Time to 20 percent death was determined and lethal threshold concentrations of 114.5 percent saturation for coho, 113.5 percent for sockeye, and 114.0 percent for steelhead were calculated. Ninety-six hour LC50 values were 120.5 percent for coho, 116.7 percent for sockeye, and 116.0 percent total gas percent saturation for steelhead. Thirty-day LC50 values were 116.2 percent for coho, 113.9 percent for sockeye, and 114.0 percent for steelhead. Severe emphysema (bubbles) occurred on fins, in the mouth, and on the head and opercles of the fish exposed to 120 percent and 115 percent saturation. Emphysema was not as severe at 110 percent but occurred in 60 percent of the coho, 20 percent of the sockeye, and 80 percent of the steelhead. Copyright (c) the American Fisheries Society 1976.