Two forms (I and II) of Glucose-6-phosphate dehydrogenase (D-Glucose-6-phosphate: NADP+ oxidoreductase E.C. 1.1.1.49) were isolated from mycelium of Aspergillus oryzae grown on glucose as sole carbon source, through ammonium sulfate fractionation, followed by ion-exchange chromatograhy. The Km values for both G6P and NADP+ were very similar, but the Vmax was nearly twofold for form II. The two isoenzymes were inhibited by NADPH competitively with NADH+, and the Ki value was minimal for isoenzymatic form I. The ratio Ki/Km was 2.5 for isoenzyme I and 7.5 for isoenzyme II. The two isoenzymatic forms were inhibited by energetic metabolites (ATP, ADP and PEP), the greater effect was caused by ATP.