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A gene for a putative glucoamylase, stg, of a hyperthermophilic archaeon Sulfolobus tokodaii was cloned and expressed in Escherichia coli. The recombinant glucoamylase (STGA) had an optimal temperature of 80oC and was extremely thermostable with a D-value of 17 hr. The pH optimum of the enzyme was 4.5. Being different from fungal glucoamylases, STGA hydrolyzed maltotriose (G3) most efficiently. Gel permeation chromatography and sedimentation equilibrium analytical ultracentrifugation analysis showed that the enzyme existed as a dimer. STGA was stable enough to hydrolyze liquefied corn starch to glucose in 4 hr at 90oC with a yield of 95%. Comparison of the kcat values for the hydrolysis and the reverse reaction at 75oC and 90oC indicated that glucose production by STGA was more efficient at 90oC than 75oC. Therefore, STGA showed great potential for application to the industrial glucose production process due to its high thermostability