Specificities of glycosidases were studied from the viewpoint of the synthesis of oligosaccharides and glycosides that are not naturally occurring. Trehalase from Lobosphaera sp. catalyzed conden-sation not only of glucose (Glc) but also of 2-deoxy glucose (dGlc), yielding trehalose and dideoxy-trehalose, respectively. Deoxy-trehalose was also synthesized in a mixture of Glc and dGlc. Activi-ties for trehalose derivatives decreased considerably because of their deoxy site. Glucoamylases from Aspergillus niger and Rhizopus niveus, and α-glucosidase from A. niger afforded deoxy-oli-gosaccharides by the condensation of dGlc, while α-glucosidases from Torula and Saccharomyces gave them in low yields. Deoxy-glucobioses with α-1, 3-, 1, 4- and 1, 6-linkages were identified as the products of glucoamylase and α-glucosidase. The hydrolysis rates of these glucobioses and the transfer action of α-glucosidase seemed to affect the product composition, especially in the early stage of the reactions, and afterwards, the 1, 6-linked saccharide accumulated predominately. Strains of Clavibacter michiganense and Flavobacterium johnsonae produced novel β-glucosidases that hydrolyzed the β-glucosyl ester linkage of steviol glycosides. Transglucosylation occurred when this linkage was degraded, ρ-Hydroxybenzoyl glucose was chemically synthesized as a substrate containing the β-glucosyl ester linkage. Several β-glucosidases, such as that from Caldocellum saccharolyticum, hydrolyzed the substrate. ρ-Hydroxybenzoyl galactose with the β-galactosyl ester linkage was also synthesized, and was examined as a substrate for β-galactosidases. The activities showed great diversity depending on the enzyme origin. The enzyme from Penicillium multicolor showed the highest activity among those tested. β-Galactosidases catalyzed galactosylation not only of a hydroxyl group but also of a thiol group in the condensation of galactose and 2-mercap-toethanol. The enzymes from A . oryzae and P . multicolor were effective for the synthesis of thio-galactoside. The usual galactoside was synthesized more rapidly than the thin-galactoside, although the latter accumulated predominately as the reaction proceeded.