We have partially purified and characterized two new thermostable exo-a-1,4-glucosidases (E.C.188.8.131.52) isolated from Geobacillus sp. A333 and thermophilic bacterium A343 strains. A333 a-glucosidase showed optimum activity at 60°C, pH 6.8 and had a value of 1.38 K m for the pNPG substrate, whereas these results were found to be 65°C, 7.0 and 0.85, respectively for A343 enzyme. Specificity for 20 different substrates and thin layer chromatography studies demonstrated that the A333 enzyme had high transglycosylation activity, and A343 had wide substrate specificity. The substrate specificity of A333 a-glucosidase was determined as maltose, dextrin, turanose, maltotriose, maltopentaose, meltotetraose, maltohexaose and phenyla-D-glycopyranoside. On the other hand, the A343 a-glucosidase mostly hydrolyzed dextrin, turanose, maltose, phenyl-a-D-glucopyranoside, maltotriose, maltotetraose, maltopentaose, isomaltose, saccharose and kojibiose by acting a-1,2, a-1,3, a-1,4 and a-1,6 bonds of these substrates. The relative activites of A333 and A343 enzymes were determined to be 83 and 92% when incubated at 60°C for 5 h whereas, the pH of 50% inactivation at 60°C for 15 h were determined to be pH 4.5/10.0 and pH 5.0/10.0, respectively. In addition, the results not only showed that both of the a-glucosidases were stable in a wide range of pH and temperatures, but were also found to be resistant to most of the denaturing agents, inhibitors and metal ions tested. With this study, thermostable exo-a-1,4-glucosidases produced by two new thermophilic strains were characterized as having biotechnological potential in transglycosylation reactions and starch hydrolysis processes.