The maltohexaose-forming alpha -amylase, of B. stearothermophilus US100, was purified to homogeneity by a combination of osmotic shack, starch adsorption and anion exchange chromatography. This enzyme has a relative molecular mass of 59 kDa. The analysis of the nucleotide sequence, of the corresponding gene, allowed the identification of a single open reading frame encoding a 549 amino acid protein, exhibiting a large homology to the other B. stearothermophilus alpha -amylases. This homology reaches a maximum with those of DY-5 and DN1792 strains with respectively 3 and 4 aa different over 549. The relatively small differences, between Amy US100 and that of DN1792 strain, take in more importance since we have demonstrated that these enzymes differ essentially by their starch hydrolysis pattern.