Emodinanthrone oxygenase, which catalyzes the oxidation of emodinanthrone to emodin, has been identified in fungus, Aspergillus terreus. The fixation of an oxygen atom at the C-10 position of emodinanthrone from molecular oxygen catalyzed by the enzyme was proved by the 18O2 incubation experiment and analyses of the product emodin by mass spectrometry and nuclear magnetic resonance. The fact that the reaction did not require any foreign electron donor suggested the involvement of internal monooxygenase. Emodinanthrone oxygenase activities were found in other microbes which produce anthraquinone related metabolites.