We have sequenced the gene encoding Bacillus stearothermophilus ATCC12016 alpha-glucosidase (alpha-D-glucoside glucohydrolase, EC 3.2.1.20) specific for nonreducing terminal alpha-1,4 bonds of maltosaccharides and alpha-glucans. The amino acid sequence of the enzyme deduced from the nucleotide sequence of the gene (1665 base pairs) consisted of 555 residues with a molecular mass of 65233. The enzyme showed 40%-57% sequence similarities to alpha-D-glucohydrolases with very different substrate specificity, such as Bacillus cereus ATCC7064 oligo-1,6-glucosidase, Bacillus thermoglucosidasius KP1006 oligo-1,6-glucosidase, Saccharomyces carlsbergensis CB11 alpha-glucosidase, Bacillus sp. F5 alpha-glucosidase, Streptococcus mutans (Ingbritt strain) dextran glucosidase, Bacillus sp. SAM1606 alpha-glucosidase and Escherichia coli ECL116 trehalose-6-phosphate hydrolase. All these enzymes had sequences equivalent to secondary elements revealed in B. cereus oligo-1,6-glucosidase by X-ray crystallography. We have suggested that the B.stearothermophilus enzyme adopts the same polypeptide folding, i.e. an (alpha/beta)(8)-barrel in the N-terminal active-site domain, as the B.cereus enzyme and other alpha-glucohydrolases.