A novel strain of Bacillus stearothermophilus was isolated from samples of a potato-processing industry. Compared to known alpha-amylases from other B. stearothermophilus strains, the isolate was found to produce a highly thermostable alpha-amylase. The half-time of inactivation of this alpha-amylase was 5.1h at 80 degrees C and 2.4 h at 90 degrees C. The temperature optimum for activity of the alpha-amylase was 70 degrees C; the pH optimum for activity was relatively low, in the range 5.5-6.0. alpha-Amylase synthesis was regulated by induction and repression mechanisms. An inverse relationship was found between growth rate and alpha-amylase production. Low starch concentrations and low growth temperatures were favourable for enzyme production by the organism. At the optimal temperature for growth, 65 degrees C, the alpha-amylase was a growth-associated enzyme. The optimal temperature for alpha-amylase production, however, was 40 degrees C, with alpha-amylase increasing from 3.9 units (U)/ml to 143 U/ml when lowering the growth temperature from 65 degrees C to 40 degrees C. Maximal alpha-amylase production in a batch fermenter run at 65 degrees C was 102 U/ml, which was 26-fold higher than in erlenmeyer flasks at 65 degrees C. The dissolved O-2 concentration was found to be a critical factor in production of the alpha-amylase.