The extent to which heat tolerance is affected by residence in a semitropical climate, season of the year, and job heat exposure, is studied in two groups of subjects: those exposed to heat in their jobs, and those working in neutral temperature jobs. Each group is tested in summer and winter. Each test involves one hour of exercise on a treadmill in a climatic chamber. Subjects are exposed to several levels of heat as measured by the following four heat indices: corrected effective temperature, wet bulb globe temperature, predicted four hour sweat rate, and the Belding-Hatch heat stress index. Results indicate that season, job heat exposure, and clothing can influence heat tolerance more than climate of residence. Subjects from both industry groups are more heat tolerant during the summer than during winter. Workers occupationally heat exposed are more heat tolerant than nonheat exposed group during heavy exercise, but not during light exercise.