The anode potential in microbial fuel cells controls both the theoretical energy gain for the microorganisms as the output of electrical energy. We operated three reactors fed with acetate continuously at a poised anode potential of 0 (R-0), -200 (R-200) and -400 (R-400) mV versus Ag/AgCl and investigated the resulting bacterial activity. The anode potential had no influence on the start-up time of the three reactors. During a 31-day period, R-200 produced 15% more charge compared to R-0 and R-400. In addition, R-200 had the highest maximal power density (up to 199 W m(-3) total anode compartment during polarization) but the three reactors evolved to the same power density at the end of the experimental period. During polarization, only the current of R-400 levelled off at an anode potential of -300 mV versus Ag/AgCl. The maximum respiration rate of the bacteria during batch tests was also considerably lower for R (-400). The specific biomass activity however, was the highest for R-400 (6.93 g chemical oxygen demand g(-1) biomass-volatile suspended solids (VSS) d(-1) on day 14). This lowered during the course of the experiment due to an increase of the biomass concentration to an average level of 578 +/- 106 mg biomass-VSS L-1 graphite granules for the three reactors. This research indicated that an optimal anode potential of -200 mV versus Ag/AgCl exists, regulating the activity and growth of bacteria to sustain an enhanced current and power generation.