A study was made of the minimum concentration of oxygen in the water necessary to maintain sockeye and chinook salmon in normal condition. It was found that when the oxygen content of the water was reduced to 3.5 p.p.m. salmon showed definite signs of distress; a reduction below 3.0 p.p.m. caused the death of some fish; a reduction below 2.5 p.p.m. brought about the asphyxiation of most of the fish in a short time. Considerable individual variation was found, some fish not dying until the oxygen concentration had dropped to 1.2 p.p.m. Some notes also are presented concerning the reaction of suckers and carp to a diminished oxygen supply.