A closed circuit respirometer is described which circulates a given volume of water through the chamber containing the fish and equilibrates the water with a small volume of air. As oxygen is consumed by the fish, its removal from the air space is indicated manometrically, and is compensated for by measuring in oxygen from a syringe. Continuous, direct measurements can be made at any intervals and for many hours. The instrument described was used for cunners weighing from 30–70 grams and for toadfish weighing 300 grams, with oxygen consumption from 1 to 12 milliliters per hour. It showed accurately the use of110milliliter oxygen per hour.