The specificity for the α-1,4- and α-1,6-glucosidic linkages varies among glycoside hydrolase family 31 α-glucosidases. This difference in substrate specificity has been considered to be due to the difference in an aromatic residue on β→α loop 1 in the catalytic domain with a (β/α)8 barrel fold; i.e., the enzymes having Tyr and Trp on β→α loop 1 were respectively described as α-1,4-specific and α-1,6-specific α-glucosidases. Schwanniomyces occidentalis α-glucosidase, however, prefers the α-1,4-glucosidic linkage, although the enzyme possesses Trp324 at the corresponding position. The mutation of Trp324 to Tyr decreased the ability for hydrolysis of the α-1,6-glucosidic linkage and formation of the α-1,6-glucosidic linkage in transglycosylation, indicating Trp324 to be closely associated with α-1,6 specificity, even if the enzyme preferred the α-1,4-glucosidic linkage. The mutant enzyme was found to catalyze the production of the branched oligosaccharide, 2,4-di-O-(α-D-glucopyranosyl)-D-glucopyranose, more efficiently than the wild-type enzyme.