Stainless steel was studied as anode for the biocatalysis of acetate oxidation by biofilms of Geobacter sulfurreducens. Electrodes were individually polarized at different potential in the range -0.20V to +0.20V vs. Ag/AgCl either in the same reactor or in different reactors containing acetate as electron donor and no electron acceptor except the working electrode. At +0.20V vs. Ag/AgCl, the current increased after a 2-day lag period up to maximum current densities around 0.7 A m(-2) and 2.4 A m(-2) with 5 mM and 10 mM acetate, respectively. No current was obtained during chronoamperometry (CA) at potential values lower than 0.00V vs. Ag/AgCl, while the cyclic voltammetries (CV) that were performed periodically always detected a fast electron transfer, with the oxidation starting around -0.25 V vs. Ag/AgCl. Epifluorescent microscopy showed that the current recorded by chronoamperometry was linked to the biofilm growth on the electrode surface, while CVs were more likely linked to the cells initially adsorbed from the inoculum. A model was proposed to explain the electrochemical behaviour of the biofilm, which appeared to be controlled by the pioneering adherent cells playing the role of "electrochemical gate" between the biofilm and the electrode surface. (C) 2008 Elsevier Ltd. All rights reserved.