Heat reactions of 20 Caucasian and 22 Bantu males were compared, first in the unacclimatized state and then in the acclimatized state. The study was conducted at temperatures of 90 F wet-bulb and 93 F dry-bulb at a work rate of 1 liter O2/min consumption. The performances of the unacclimatized Bantu were superior to those of the Caucasians. All 22 Bantu completed the 4-hr experiment, while 10 Caucasians failed. The mean rectal temperature of the Bantu was significantly lower than that of the Caucasians, but not the mean heart rate and mean sweat rate. When both groups were highly acclimatized all men from both groups completed the 4-hr experiment, and their reactions to heat were significantly different from their reactions in the unacclimatized state. Sweat rates, particularly, increased very much. The differences between the two highly acclimatized groups in rectal temperatures, heart rates, and sweat rates (except the 4th hr) were not significant. Although superior in the unacclimatized state, the Bantu does not appear to have an inherent advantage in the ability to regulate the body temperature. Caucasians versus Bantu in reactions to heat; physiological reactions in exposure to heat Submitted on August 19, 1963