Glucoamylase I of black Aspergillus was purified by (NH_4)_2SO_4 precipitation, ethanol fractionation, ion exchange chromatography on DEAE-cellulose, preparative isoelectric focusing and Sephadex G-100 gel filtration. The enzyme thus purified was found to contain no α-amylase and appeared to be homogeneous in polyacrylamide gel electrophoresis. The isoelectric point of glucoamylase I was at pH 3.4. The optimum conditions for its action on boiled soluble starch were at 60℃ and pH 4.5. The enzyme was quite stable at pH 4.0-5.0 and temperature up to 40℃. It had a strong debranching activity (0.56) and it hydrolyzed about 75 % of soluble starch but glycogen and p-limit dextrin from glycogen almost completely. It could be almost completely adsorbed onto raw starch and was active in raw starch digestion. Maximum digestion of raw starch by this enzyme occurred at pH 3.4. Glucoamylase I contained about 7.6 % carbohydrate.