This study examined the independent and combined importance of aerobic fitness and body fatness on physiological tolerance and exercise time during weight-bearing exercise while wearing a semipermeable protective ensemble. Twenty-four men and women were matched for aerobic fitness and body fatness in one of four groups (4 men and 2 women in each group). Aerobic fitness was expressed per kilogram of lean body mass (LBM) to eliminate the influence of body fatness on the expression of fitness. Subjects were defined as trained (T; regularly active with a peak aerobic power of 65 ml · kg LBM¹ · min¹) or untrained (UT; sedentary with a peak aerobic power of 53 ml · kg LBM¹ · min¹) with high (High; 20%) or low (Low; 11%) body fatness. Subjects exercised until exhaustion or until rectal temperature reached 39.5°C or heart rate reached 95% of maximum. Exercise times were significantly greater in T_Low (116 ± 6.5 min) compared with their matched sedentary (UT_Low; 70 ± 3.6 min) or fatness (T_High; 82 ± 3.9 min) counterparts, indicating an advantage for both a high aerobic fitness and low body fatness. However, similar effects were not evident between T_High and UT_High (74 ± 4.1 min) or between the UT groups (UT_Low and UT_High). The major advantage attributed to a higher aerobic fitness was the ability to tolerate a higher core temperature at exhaustion (the difference being as great as 0.9°C), whereas both body fatness and rate of heat storage affected the exercise time as independent factors.