Current outputs of microbial fuel cells (MFCs) are too low for most perceived practical applications. Most efforts for further optimization have focused on modifications of fuel cell architecture or electrode materials, with little investigation into the properties of microorganisms that are most essential for maximal current production. Geobacter sulfurreducens produces the highest current densities of any known pure culture; is closely related to the Geobacter species that often predominate in anode biofilms harvesting electricity from organic wastes; and produces highly conductive anode biofilms. Comparison of biofilm conductivities and current production in different strains of G. sulfurreducens revealed a direct correlation between biofilm conductivity and current density. Electrochemical impedance spectroscopy measurements demonstrated that higher biofilm conductivity not only reduced resistance to electron flow through the biofilm, but also lowered the activation energy barrier for electron transfer between the biofilm and the anode. These results demonstrate the crucial role of biofilm conductivity in achieving high current density in MFCs and suggest that increasing biofilm conductivity can boost MFC performance.