Abstract: An extracellular a-amylase from an extreme thermophile, Thermus sp., was highly purified by affinity absorption on starch granules. SDS-PAGE showed a single band for the purified enzyme, with an apparent molecular weight of 59000. The optimum pH and temperature for the enzyme action on starch was 5.5_6.5 and 70°C, respectively. The enzyme randomly attacked the bonds in the inner region of the starch and produced various maltooligosaccharides. The minimum length of maltooligosaccharide cleaved by this enzyme was maltohexaose. The enzyme activity was strongly inhibited by the addition of Cu2+ and Fe2+ ions. The enzyme belonged to the EDTA-sensitive a-amylase group, but its activity was not stimulated by the presence of Ca2+ ions.