The heat ,resistance of Bacillus stearothermophilus spores was determined in water vapor, aqueous solutions of NaCl, LiCl, glucose and glycerol and whole egg powder, fish protein concentrate (FPC) and wheat flour at different water activities. In water vapor and glycerol solutions, resistance was maximal at low, but not zero, water activity (a,). In NaCl and glucose solutions, only small variations in resistance occurred with decreasing a,, whereas in LiCl solutions, resistance showed a minimum around a, 0.5. When heated in egg powder, FPC and wheat flour, the spores showed maximal resistance at intermediate water activities in accordance with the results in water vapor and glycerol solutions.