A response surface model was developed to describe the effects of temperature (35 to 55 degrees C), pH (3.5 to 5.5), and water activity (a(w), 0.960 to 0.992) on germination of Alicyclobacillus acidoterrestris spores. Germination and growth or viability loss depended, to varying extents, on the interactions among the independent variables and the complexity of the medium. In particular, maximum growth was achieved at temperatures between 35 and 42 degrees C and at pH values from 3.5 to 4.5. The model was validated against data not used in its development. Bias factors of 0.999 and 0.817 for 2- and 7-day models, respectively, were obtained, indicating that the models were "fail safe." Results indicated that the model provided reliable predictions of growth of A. acidoterrestris spores.