An energy‐economic use of reaction heat is possible only if thermophilic microorganisms convert carbon substrates into biomass at temperatures above 65°C. The thermophilic bacterium Bacillus stearothermophilus TP5 was isolated from a man‐made hot water reservoir. The highest specific growth rate of 1.6 h−1 was obtained in continuous cultivation (chemostat) on a glucose‐containing nutrient medium at 68°C, pH 6.8 and normal pressure. The highest values for the biomass production rate (3.2 g dm−3 h−1) and the biomass yield coefficient (0.39 gg−1) were obtained at a dilution rate of 1.3 h−1 (about 80% of the highest specific growth rate) under the above‐mentioned conditions. The biomass production rate of the thermophilic bacterium was higher in the chemostat than in the semicontinuous process at normal pressure. The biomass produced in the chemostat at a dilution rate of 0.4 h−1 contained 71% of protein and 8% of RNA and had an ash content of 6%. The optimum and maximum temperatures of growth were changed by raising the pressure inside the reactor. As a prerequisite for the energy‐economic use of the reaction heat, the specific heat production was determined by using a special calorimeter. The ratio of heat production to oxygen consumption (oxy‐caloric coefficient) was found to be 13.6 kJ g−1, which is close to the theoretical value deduced from mass‐ and energy‐balance theory.