PURPOSE: Treadmill testing and cycle ergometry are the most common modes of exercise testing for assessing maximal aerobic capacity (VO2max). Most sedentary subjects and trained runners have higher VO2max measurements during treadmill testing compared to cycle testing, suggesting mode of exercise affects VO2max. However, trained cyclists attain higher VO2max values on cycle ergometry than treadmill testing due to the specific adaptations of cycle training. Front-crawl swimming is a dynamic exercise involving both upper and lower limbs. The purpose of this study was to determine if trained swimmers have higher VO2max values when swimming compared to running. METHODS: Eight trained swimmers (3 M, 5 F; 21.6 ± 2.9 years) performed VO2max testing on a treadmill and in a swim flume. For the flume test, subjects breathed through a 2-way non-rebreathing snorkel that collected their expired breath for analysis. For the treadmill test, subjects used a mouthpiece, nose clip, and 2-way non-rebreathing valve. For both modes of exercise, the subjects’ expired air was collected into a mixing chamber and analyzed by a metabolic cart. The subjects exercised at increasing intensities until volitional fatigue on both tests. Blood lactate was assessed before and after each test. Metabolic measurements and heart rates (HR) were measured continuously and reported as 10-s averages. RESULTS: The subjects had higher VO2max values on the treadmill than in the swim flume (56.2 ± 7.8 vs. 50.6 ± 11.5 ml kg-1 min-1, p = 0.034). At VO2max, minute ventilation, tidal volume, and respiratory frequency were all higher on the treadmill than in the flume. Respiratory exchange ratio and HR were significantly higher following the treadmill test and post-run lactate tended to be higher on the treadmill test (p = 0.055). However, oxygen pulse (VO2/HR), an index of cardiac stroke volume, did not differ between tests, as both VO2 and HR were lower while swimming. CONCLUSIONS: Results from the study suggest that running elicits a greater cardiovascular demand than swimming even in trained swimmers. This may be due to greater involvement of the larger leg muscles in running than in swimming. Future studies should examine the other three competitive swimming strokes, as they are less efficient than the front crawl and more reliant on the lower body for propulsion.