Diacetyl in a fermented liquor is determined by WHITE, KRAMPITZ and WERKMAN's method (Arch. Biochem., 9, 229, 1946), using a photoelectric spectrophotometer for the production of yellow colour. By this method most Aspergilli and especially a strain of Asp. oryzae COHN (Kagawa-Ken Shoyu Lab. No. Y) were proved to produce, diacetyl, acetoin and 2, 3-butylenc glycol. The optimum producing conditions by this strain were as follows: temp. 33°; pH 6.5; C-source, mannose; N-source, KN0₃; C:N, 75:1. Specific rotatory power [α]^20°_D of the acetoin produced by surface culture for 13 days was -6.76. Fig. 3 shows the periodical transition curves of these three compounds produced by surface and shaking cultures. It gives important suggestions on our enzymatic studies in the future.