The objective of this study is to compare the caloric expenditure and oxidative sources of underwater treadmill running and land-based treadmill running at maximal and submaximal levels. Underwater running has emerged as a low load bearing form of supplementary training for cardiovascular fitness, as a way to promote recovery from strenuous exercise while maintaining aerobic fitness, and as a way to prevent injury. Prior studies have reported conflicting results as to whether underwater treadmill running elicits similar cardiorespiratory responses to land-based running. It is important to further investigate the similarities and differences between the two to determine if underwater running is as efficient as land-based running for maintenance of fitness and for rehabilitative purposes. Purpose: To compare the caloric expenditure and oxidative sources of underwater treadmill running and land treadmill running during both maximal treadmill trials to exhaustion and during 30 minute submaximal treadmill trials. Methods: 11 volunteer experienced male triathletes, ages 18-45 were recruited as participants. Each completed 6 trials total which included a maximal and submaximal oxygen consumption trial for each of three conditions: running on a water treadmill with AQx® water running shoes, running on a water treadmill without shoes, and running on a land-based treadmill. Data analysis: Data was analyzed using repeated measures ANOVAs, paired t-tests, pairwise comparisons with bonferroni adjustments, and descriptive statistics were reported. Results: For maximal oxygen consumption trials VO2, RPE, RER, and BP were not significantly different between modalities. Maximal HR was found to be significantly different between modalities, and was shown to be greater on land than in the water. For submaximal VO2, trials HR, RPE, RER, and post BP were not found to be significantly different between modalities. Average VO2, total calories expended, and pre systolic BP were found to be significantly different, and were shown to be greater on land than in water. Conclusions: While maximal exertion running on underwater treadmills seems to elicit similar cardiorespiratory responses to running on land-based treadmills, differences were seen at submaximal exertion levels. It remains unclear whether underwater treadmill running can elicit similar training stimuli as land running at submaximal levels.