This research project was designed to compare energy expenditures during water-immersion and ambient-air states at three external work rates of 50, 100, and 150 W. Eleven participants were tested on two separate occasions on a bicycle ergometer in water-immersion and ambient-air environments for 3 to 4 min at 50 rpm. Oxygen consumptions and heart rates were monitored continuously throughout both sessions. Perceived exertion was assessed at each work rate during the final 30 s of the sampling period. Water-immersion produced higher oxygen consumption values (2.21 ±0.20 vs. 1.00 ±0.14 L min-1 at 50 W; 2.64 ±0.33 vs. 1.45 ±0.12 L min-1 at 100 W; and 2.86 ±0.41 vs. 2.00 ±0.16 L min-1 at 150 W). Heart rates were greater in water (142.70 vs. 97.90 at 50 W; 152.73 vs. 113.14 at 100 W; and 163.82 vs. 135.09 at 150 W). Perceived exertion scores were higher in water (11.7 vs. 8.5 at 50 W; 15.0 vs. 10.6 at 100 W and 17.2 vs. 12.7 at 150 W). Energy expenditure rates were greater in the water-immersion environment (769 vs. 348 W at 50 W; 919 vs. 505 W at 100 W; 995 vs. 696 W at 150 W). It was concluded that at a constant external load, water-immersion produces greater oxygen consumption and heart rate responses compared to values assessed in an ambient-air state. This difference reflects the external load required to move the viscous liquid instead of air.