A stable biofilm of Penicillium chrysogenum in a slab geometry was characterized with respect to penicillin production at various sugar and oxygen concentrations by studies using a novel bench scale bioreactor. The biofilm was submerged in aerated liquid media, mounted vertically in a tube and oriented so that the liquid media and sparged gas bubbles flow uniformly over the two slab faces. The conditions necessary for definite oxygen and sugar limitations were found by operating under various bulk nutrient concentrations. The periodic determination of total biofilm volume (rate of growth) and dry cell weight provided additional information. Definite oxygen mass transfer limitations were found to exist in the biofilm. These limitations could be overcome by increasing the oxygen supply rate to the biofilm.