$alpha$-Glucosidase of an extreme thermophile, Thermus caldophilus GK24 (TcaAG), was purified 80-fold from cells to a homogeneous state and characterized. The enzyme exhibited optimum activity at pH 6.5 and $90^{circ}C$, and was stable from pH 6.0 to 85 and up to $90^{circ}C$. The enzyme had a half-life of 85 minutes at $90^{circ}C$. An analysis of the substrate specificity showed that the enzyme hydrolyzed the non-reducing terminal unit of $alpha$-1,6-glucosidic linkages of isomaltosaccharides and panose, $alpha$-1,3-glycosidic bond of nigerose and turanose, and $alpha$-1,2-glycosidic bond of sucrose. The gene encoding the TcaAG was cloned, sequenced, and sequenced in E. coli. The nucleotide sequence of the gene encoded a 530 amino acid polypeptide and had a G+C content of 68.4% with a strong bias for G or C in the third position of the codons (93.6%). A sequence analysis revealed that TcaAG belonged to the $alpha$-amylase family. We suggest that this monomeric, thermostable, and broad-acting $alpha$-glucosidase is a departure from previously exhibited specificities. It is, therefore, a novel $alpha$-glucosidase.