In this study, variations in corrosion potential and polarization resistance of thin-film gold electrodes as a result of the growth of Pseudomonas fluorescens biofilms on them are presented. The growth of the volumetric cell fraction of biofilms, as determined by optical sectioning and digital image analysis of phase-contrast images, was found to be exponential during at least 10 hours of incubation. As a consequence of biofilm growth, an exponential decay of the corrosion potential of gold was observed. Most importantly, an increase in polarization resistance of the interface was observed following a strong linear dependence on the mean thickness of biofilms (r = 0.997), as a consequence of oxygen consumption and diffusion limitations. The results presented indicate that the measurement of polarization resistance may be a suitable technique that could be applied easily in industrial or biotechnological systems for monitoring the formation of biofilms. (C) 2003 Wiley Periodicals, Inc.