The purpose of this project was to determine whether mild heat stress induced by wearing a wet suit while swimming in relatively warm water (25.4± 0.1°C) increases the risk of heat injury during the cycling and running stages of an International distance triathlon in a hot and humid environment (32°C and 65% RH). Five male triathletes randomly completed two simulated triathlons (swim = 30 min; bike = 40 km; run = 10 km) in the laboratory using a swimming flume, cycle ergometer, and running treadmill. In both trials, all conditions were identical, except for the swimming portion in which a neoprene wet suit was worn during one trial (WS) and a swimming suit during the other (SS). The swim portion consisted of a 30-min standardized swim in which oxygen consumption (˙VO2) was replicated, regardless of WS or SS. During the cycling and running stages, however, the subjects were asked to complete the distances as fast as possible. Core temperature(Tc) was not significantly different between the SS and WS trials at any time point during the triathlon. However, mean skin temperature(Tsk) and mean body temperature (Tb) were higher (P< 0.05) in the WS at 15 (Tsk = +4.1°C, Tb = +1.5°C) and 30 min (Tsk = +4°C, Tb = +1.6°C) of the swim. These Tsk and Tb differences were eliminated by 15 min of the cycling stage and remained similar (P > 0.05) through the end of the triathlon. Moreover, there were no differences (P > 0.05) in˙VO2, heart rate (HR), rating of perceived exertion (RPE), or thermal sensation (TS) between the WS and SS. Additionally, no significant differences were found in cycling (SS = 1:14:46 ± 2:48 vs WS = 1:14:37± 2:54 min), running (SS = 55:40 ± 1:49 vs WS = 57:20 ± 4:00 min), or total triathlon times (SS = 2:40:26 ± 1:58 vs WS = 2:41:57 ± 1:37 min). These data indicate that wearing a wet suit during the swimming stage of an international distance triathlon in 25.4°C water does not adversely affect the thermoregulatory responses of the triathlete on the subsequent cycling and running stages.