A steady-state model was developed to predict the biofilm thickness, microbial distribution and substrate fluxes based on the microbial interactions in multispecies biofilms under the action of an electric field. The experimental evaluations were carried out in completely mixed biofilm reactors where nitrifying bacteria compete with glucose-utilizing heterotrophs. The experimental results showed that the model successfully describe the competition of the two types of bacteria. The biofilm thickness and flux of NH4+ increases with increasing electric field strength, whilst the flux of glucose slightly decreases. In addition, increasing the current density not only forces autotrophs and heterotrophs to move into the deeper biofilm but also decreases the biofilm thickness and substrate fluxes. (C) 2002 Elsevier Science Ltd. All rights reserved.