To better characterize the athletes who participate in ultraendurance triathlons, 14 triathletes in training for the Hawaii IRONMAN triathlon were studied. A physical and physiological profile was developed from anthropometric measurements and oxygen uptake during maximal exercise on a treadmill, cycle ergometer, and arm ergometer. A comparison of the maximal values among exercise modes and between males and females was made. A comparison of height, weight, and percent body fat of these triathletes with elite athletes from the sports of swimming, cycling, and running showed the physique of triathletes to be most similar to that of cyclists. Oxygen uptake at maximal exercise was, for males and females, respectively: 68.8 ml X kg-1 X min-1, 65.9 ml X kg-1 X min-1 on the treadmill; 66.7 ml X kg-1 X min-1, 61.6 ml X kg-1 X min-1 on the cycle ergometer; and 49.1 ml X kg-1 X min-1, 39.7 ml X kg-1 X min-1 on the arm ergometer. When comparing the highest oxygen uptake attained at maximal exercise in any one of the three exercise modes, the male triathletes are comparable to swimmers, but have a lower aerobic capacity than cyclists or distance runners. The female triathletes studied were able to attain oxygen uptake values greater than those previously reported for female athletes.