An Absidia sp. 28 strain was shown to have higher activity of 11 alpha-hydroxylation using 16 beta-methyl-17 alpha,21-dihydroxy-1,4-pregnadiene-3,20-dione as a substrate. It was found that 21-acetylization of substrate could increase 11 alpha-hydroxylation rate conspicuously. The yield of the 11 alpha-hydroxylation by this strain was 73% in the conversion of 0.5% concentration of 21-acetated substrate under optimum conditions.