To apply membrane-surface liquid culture (MSLC), reported in previous papers (J. Ferment. Bioeng., 80 : 35-40, 41-46, 1995) to large-scale production, a novel bioreactor was constructed in which molds were grown on the outer surface of a cylindrical porous membrane facing the air with liquid medium running down along its inner surface. We cultivated Aspergillus oryzae NRRL484 to produce kojic acid as a model secondary metabolite. An SE20 membrane composed of polysulfone with a nominal pore diameter of 0.2 mu m (Fuji Photo Film Co.) was found to be most suitable based on the facts that the mass transfer through the membrane had no appreciable effect on the kojic acid production and the mycelia did not pass through the membrane pores. Kojic acid was produced at higher levels than in shaking flask culture. The highest kojic acid concentration in the batch MSLCs using the cylindrical membrane module with an active surface area of 220 cm(2) was 14, 45, and 60 mg/ml for glucose concentrations in the medium of 5, 10, and 20%, respectively. In the shaking flask cultures using 10 and 20% glucose, the maximum kojic acid concentrations were 24 and 22 mg/ml, respectively. In the MS;LC, the liquid medium was quite clear without any contamination of cells. Furthermore, the mold was highly stable against autolysis, and continuous cultivation with holding tells on the membrane surface could be conducted for over 70 d. When the average feed rate of the medium containing 10% glucose and 0.1% yeast extract was 30 ml/d, the steady-state kojic acid concentration at the outlet of the MSLC apparatus was in the range of 45-50 mg/ml.