With a few simple modifications, a conventional row-bicycle ergometer could be made suitable for underwater use. By attaching three sizes of fins to the pedal cranks of the ergometer, work load in water could be achieved to an oxygen consumption (VO₂) level range of 400 to 2000 ml/min. The preferable range of pedaling speeds was 30-40 rpm to maintain a constant speed for a prolonged time. When using three different fins VO₂ was found to be: VO₂=254.3+4.50n-0.12n²+0.0030n³ for no fins, VO₂=250.1+8.30n-0.32n²+0.0062n³ for small fins, VO₂=254.0+6.10n-0.24n²+0.0068n³ for medium fins, and VO₂=256.4+13.77n-0.72n²+0.0131n³ for large fins, when n is the number of pedaling rate. Major advantages of this modified ergometer for underwater exercise are 1) the modification is simple and the size of the device is small enough to place in a tank of a climatic chamber, 2) the ergometer can be biologically calibrated and used for a wide range of exercise oxygen uptakes, and 3) a subject can exercise in a comfortable upright position.