The oxygen consumption of fish and other aquatic animals can be determined in the gas phase of a closed system provided the water is thoroughly equilibrated with the gas phase. The present respirometer employs a simpler and more efficient equilibration system than the earlier described model [Scholander, Haugaard, and Irving, Rev. Sci. Inst. 14, 48 (1943).] and permits a continuous recording of the oxygen consumption over a considerable range. The apparatus is well suited to be set up in banks and has been used extensively in temperature studies on arctic and tropical fishes and crustaceans.