The thermostable beta -glycosidase from Thermus thermophilus, cloned and overexpressed in Escherichia coli was used to catalyze the transfer Of beta -D-glucosyl and beta -D-fucosyl from the corresponding p-nitrophenyl-beta -D-glycopyranosides to a hydroxyl group of glucose in the synthesis Of beta -D-glucosyl-D-glucopyranoses and beta -D-fucosyl-D-glucopyranoses. The yields in disaccharides produced under conditions of non-initial velocity were very attractive and the formation of the beta (1-3) linked disaccharides was largely favored. The enzyme could constitute a valuable biocatalyst for the synthesis of disaccharides involving such structures as, for example, Bifidus factors.