A thermostable glucoamylase (TtcGA) from Thermoanaerobacter tengcongensis MB4 was successfully expressed in Escherichia coli. The full-length gene (2112 bp) encodes a 703-amino acid polypeptide including a predicted signal peptide of 21 residues. The recombinant mature protein was partially purified to 30-fold homogeneity by heat treatment and gel filtration chromatography. The mature protein is a monomer with the molecular weight of 77 kD. The recombinant enzyme showed maximum activity at 75 degrees C and pH 5.0. It is the most thermostable bacterial glucoamylase described to date with nearly no activity loss after incubation at 75 degrees C for 6 h. TtcGA can hydrolyze both alpha-1, 4- and alpha-1, 6-glycosidic linkages in various alpha-glucans. It showed preference for maltooligosaccharides over polysaccharides with specific activity of 80 U/mg towards maltose. Kinetic studies revealed that TtcGA had the highest activity on maltooligosaccharide with four monosaccharide units. The cations Ca2+, Mn2+, Co2+, Mg2+, and reducing agent DTT showed no obvious effects on the action of TtcGA. In contrast, the enzyme was inactivated by Zn2+, Pb2+, Cu2+, and EDTA.