Acid-stable alpha-amylase (asAA) produced by Aspergillus kawachii (IFO4308) during shochu-koji production was investigated. Two types of asAA (asA-1 and asA-2) were purified from barley shochu-koji and their molecular weights mere estimated to be 108,000 and 58,000, respectively. The characteristics of the adsorption of asA-1 and asA-2 onto raw starch were different, In barley shochu-koji production, the asA-1 activity increased gradually, but decreased after 24 h incubation. In contrast, the asA-2 activity increased with incubation time, The ratio of asA-1 to total asAA activity (A1/AA ratio) was influenced by temperature, asA-1 was incubated with the acid protease of A. kawachii, and the mixture mas subjected to SDS-PAGE analysis. The band corresponding to asA-1 disappeared at 12 h into the incubation time, and an unknown band appeared at this time, The unknown protein possessed almost the same characteristics with respect to alpha-amylase activity and molecular weight as did asA-2, From these results, acid-stable alpha-amylase is concluded to have many forms similar to the glucoamylases of Aspergillus awamori var, kawachi and it is thought that it is formed during shochu-koji production.