Through their ability to directly transfer electrons to electrodes, Geobacter sp. are key organisms for microbial fuel cell technology. This study presents a simple method to reproducibly select Geobacter-dominated anode biofilms from a mixed inoculum of bacteria using graphite electrodes initially poised at -0.25, -0.36 and -0.42 V vs. Ag/AgCl. The biofilms all produced maximum power density of approximately 270 mW m(-2) (projected anode surface area). Analysis of 16S rRNA genes and intergenic spacer (ITS) sequences found that the biofilm communities were all dominated by bacteria closely related to Geobacter psychrophilus. Anodes initially poised at -0.25 V reproducibly selected biofilms that were dominated by a strain of G. psychrophilus that was genetically distinct from the strain that dominated the -0.36 and -0.42 V biofilms. This work demonstrates for the first time that closely related strains of Geobacter can have very different competitive advantages at different anode potentials. (C) 2013 Elsevier Ltd. All rights reserved.