ABSTRACTData are presented supporting a new model of spore populations during isothermal and dynamic lethal heat treatments. the model incorporates activation, injury, and preliminary inactivation of less‐resistant fractions as well as the usual predominant inactivation. Rate constants of those transformations were determined experimentally forBacillus subtilisstrain A and were found to vary with temperature according to Arrhenius equations. Model generated and experimental isothermal survivor curves compared well. Comparison of model and experimental survivor curves for this species in a time‐varying temperature regime showed the model to be a potentially good predictor of survivors during dynamic lethanl heat treatment. the new model could be particularly important in simulating sterilization and pasteurization processes, especially short duration UHT treatments, and microbiological validation of arbitrary, dynamic thermal proces