1. Body mass is a key determinant of diving performance in endotherms. In air‐breathing ectotherms however, this paradigm occurs with considerably less force. Here, through remote recordings of dive behaviour over a wide size range (5–42 kg body mass, n = 17) of freshwater crocodiles (Crocodylus johnstoni), we demonstrate why body mass is such a poor determinant of dive duration for ectothermic divers. 2. Crocodiles were released into the wild with a time‐depth‐recorder attached to their dorsal scutes, and a movement activated radio‐tag attached to their tail. Over 15 days, 652·6 ± 58·4 (mean ± SE, n = 17) dives were recorded, with all individuals exhibiting two specific dive‐types. These were, a resting‐dive (62·7 ± 5·4% of total dive no.), where no activity occurred during the dive, and an active‐dive (37·1 ± 6·3% of total no.) associated with swimming. 3. The durations of resting‐dives (∼12 min) were similar for all crocodiles. Smaller crocodiles (6·3 ± 0·7 kg, mean ± SE, n = 9) exhibited a significant correlation between dive duration and post‐dive surface‐interval, whilst larger crocodiles (17·9 ± 3·75 kg, mean ± SE, n = 8) did not. This demonstrated that aerobic dive duration was mass‐specific during resting‐dives, but other mass specific factors, presumably ecological, determined dive duration. 4. The durations of active‐dives were never >1 min, showed no relationship with body mass and no correlation with the post‐dive‐surface interval. In crocodiles, aerobic metabolic scope is independent of body mass but anaerobic capacity is mass dependent, suggesting that active‐dive duration was determined by sustained activity and dives were terminated before anaerobic metabolism became significant. 5. All individuals showed similar diel phase shifts in dive duration, type and depth, illustrating the overwhelming influence of the external environment on dive behaviour. Dive durations which resulted in significant anaerobic debt occurred rarely, but were undertaken in response to a potential threat. 6. Body mass was a poor predictor of diving in C. johnstoni because the external environmental and ecological factors exerted a greater influence on dive duration than oxygen reserves.