In order to mise the yield of hydrocortisone (HC) from cortexolone-21-acetate (RSA), 11 beta-hydroxylase-producing Absidia orchidis was entrapped in calcium alginate gel and the bioconversion was conducted in cosolvent-containing media. The optimal immobilization procedure for Absidia orchidis was determined. The factors of influence on the conversion were studied in detail. The optimal concentrations of the cosolvent, detergent, and substrate in the reaction system were investigated. The optimal pH and temperature of the biotransformation were also determined. It was found that the ratio of HC(11 beta-isomer) and 11 alpha-hydroxyl hydrocortisone (alpha-HC) in the products was 2.2-2.5 and the yield of HC reached 59-61% in a cosolvent-containing system. This can be compared with the ratio of HC and alpha-HC(HC/alpha-HC) of 0.9 and HC yield of 40% in an aqueous system. Stability studies showed that the immobilized cells can he used for six times without any significant decrease in activity. The results reported in this paper show a high potential in industrial production of HC.