Introduction: During military fin swimming, we suspected that oxygen uptake ( O2) and pulmonary ventilation ( E) might be much higher than expected. In this framework, we compared these variables in the responses of trained military divers during land cycling and snorkeling exercises. Methods: Eighteen male military divers (32.3 ± 4.2 years; 178.0 ± 5.0 cm; 76.4 ± 3.4 kg; 24.1 ± 2.1 kg m-2) participated in this study. They performed two test exercises on two separate days: a maximal incremental cycle test (land condition), and an incremental fin swimming (fin condition) in a motorized swimming flume. Results: The respective fin and land O2max were 3,701 ± 39 mL min-1 and 4,029 ± 63 mL min-1 (p = 0.07), these values were strongly correlated (r 2 = 0.78 p < 0.01). Differences in O2 between conditions increased relative to l; O2max (r 2 = 0.4 p = 0.01). Fin E values were significantly lower than land E values (p = 0.01). This result was related to both the significantly lower fin Vt and f (p < 0.01 and <0.04, respectively). Consequently, the fin E / O2 ratios were significantly lower than the corresponding ratios for land values (p < 0.01), and the fin and land E were not correlated. Other parameters measured at exhaustion-PaO2, PaCO2, and SO2 - were similar in fin and land conditions. Furthermore, no significant differences between land and fin conditions were observed for peak values for heart rate, blood lactate concentration, and respiratory exchange ratio R. Conclusion: Surface immersion did not significantly reduce the O2 in trained divers relative to land conditions. As long as O2 remained below O2 , the E values were identical in the two conditions. Only at O2 was E higher on land. Although reduced by immersion, E provided adequate pulmonary gas exchange during maximal fin swimming.