An analytical reaction engineering model, recently presented by van Ede et al. (1992), which describes formation of inhibiting products associated with the growth of immobilized biofilms, is extended to describe simultaneous production associated with biofilm maintenance. The model accounts for both the metabolic rate controlling behaviour of an inhibitory product in the biofilm, and the effect of diffusion limitation in the transport of this product or a substrate, on the biofilm thickness. Simple criteria are presented to check the applicability of the model in the case of true Monod kinetics. As a potentially industrially important example, ethanol production by Zymomonas mobilis is treated.