Steady-state performance of a three-phase fluidized-bed biofilm fermentor system, which is used for production of penicillin, is analyzed by considering two ideal contacting patterns, i.e., complete-mixed and plug-flow ones. At steady state, the biofilm thickness is kept constant by assuming that the microbial growth is balanced with cell decay processes. From the simulation analyses, it was found that a complete-mixed contacting pattern gives superior performance to plug-flow one in both outlet product concentration and specific productivity of the reactor system. The inhibitory effect of the carbon source, glucose, is found to be less pronounced in the former configuration, and this fact is considered an important criterion for the design purposes. Optimum biofilm thickness for the biofilm fermentor system was found to be a function of various operationg parameters such as inlet substrate concentration and oxygen transfer capacity of the system. It should therefore not be determined directly from the penetration depth of a limiting nutrient, but from information on the interactions between reactor productivity and those operating variables.