Two sequence-homologous alpha-amylases from B. amyloliquefaciens and B. licheniformis were studied with respect to their stability against heat denaturation and were compared with respect to common structure-stabilizing principles. The investigated alpha-amylases were isolated from culture broth of B. amyloliquefaciens and B. licheniformis. The molecular parameters (molecular weight and isoelectric point) are similar. The thermostability was determined by changes of the protein structure (changes of the fluorescence emission spectra). At pH 5.0 the thermostable alpha-amylase from B. licheniformis showed a rate of denaturation which was achieved by the thermolabile alpha-amylase from B. amyloliquefaciens at a temperature 15 degrees lower. The alpha-amylase from B. licheniformis exhibits a marked stability also at the alkaline pH-range in contrast to the alpha-amylase from B. amyloliquefaciens. From measurements in the presence of EDTA and Ca2+ follows that both enzymes are stabilized by binding of calcium ions. An analysis of preferred amino acid exchanges between the two sequence-homologous alpha-amylases showed correspondences and differences to the well-known diagram of ARGOS. Possibly an increased thermic stability can already be achieved by special amino acid exchanges without significant changes in the protein structure.