The production process for Arthrobacter simplex with high specific hydrocortisone Delta(1)-dehydrogenase activity was improved by medium optimization in parallel shake flask experiments. Using a genetic algorithm, the concentrations of 12 medium components (mineral salts and amino acid) were optimized within 144 experiments to give maximum specific hydrocortisone Delta(1)- dehydrogenase activity and biomass yield (full experimental plan : 1.5 x 10(14) experiments). The specific hydrocortisone Delta(1)-dehydrogenase activity was improved by a factor of 4.5 to 1800 U g(-1). With the optimized medium composition in a stirred tank reactor for batch production of A. simplex, a specific hydrocortisone Delta(1)-dehydrogenase activity of 2000-3000 U g(-1) dry biomass was achieved depending on harvest time (previously reported data : 18-37 U g(-1)).