An extracellular alpha-glucosidase from the thermophilic archaebacterium Thermococcus strain AN1 was purified 875-fold in five steps (Hiload Q-Sepharose, phenyl Sepharose, HPHT-hydroxyapatite, gel filtration and Mono Q chromatography) with a yield of 4%. It is a monomer with a molecular mass of about 60 kDa and a pI around 5. At 98 degrees C, the purified enzyme in buffer has a half-life around 35 min, which is increased to around 215 min in presence of 1% (w/v) dithiothreitol and 1% (w/v) BSA. Dithiothreitol (1%, w/v) and BSA (0.4%, w/v) also substantially increase the enzyme activity. The Km at 75 degrees C is 0.41 mM with pNP-alpha-D-glucopyranoside as substrate. The substrate preference of the enzyme is: pNP-alpha-D-glucoside > nigerose > panose > palatinose > isomaltose > maltose and turanose. No activity was found against starch, pullulan, amylose, maltotriose, maltotetraose, isomaltotriose, cellobiose and beta-gentiobiose. A variety of techniques including immobolization (e.g., on epoxy and glass beads), chemical modification (cross- and cocross-linking) and the use of additives (including polyhydroxylic molecules, BSA, salts, etc.) were applied to enhance stability at temperatures above 100 degrees C. The half-life could be increased from about 4 min at 100 degrees C to 30-60 min at 130 degrees C in presence of 90% (w/v) sorbitol, 1% (w/v) dithiothreitol and 1% (w/v) BSA, and by cross-linking with BSA in the presence of 90% (w/v) sorbitol. The stabilized enzyme showed good activity at 130 degrees C.