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A mutant strain (KL-38) of Aspergillus oryzae was obtained by UV irradiation. Phytase activity of KL-38 in molded rice (koji rice) was about 2.7-fold of that obtained from the parent strain (BP-1). Phytase activity of KL-38 in the submerged culture was similar to that of BP-1. Two types of phytase were produced from koji culture: phytase I (Phy I) was produced during incubation of both koji and submerged cultures, and phytase II (Phy II) was obtained only from koji culture. Phy II production was increased in KL-38 compared with BP-1, whereas the production of Phy I was similar for both KL-38 and BP-1. This finding indicates that A. oryzae has at least two types of phytase isozyme.