Power generation in microbial fuel cells (MFCs) is a function of the surface areas of the proton exchange membrane (PEM) and the cathode relative to that of the anode. To demonstrate this, the sizes of the anode and cathode were varied in two-chambered MFCs having PEMs with three different surface areas (A(PEM)=3.5, 6.2, or 30.6 cm(2)). For a fixed anode and cathode surface area (A(An)=A(Cat)=22.5 cm(2)), the power density normalized to the anode surface area increased with the PEM size in the order 45 mW/m(2) (A(PEM)=3.5 cm(2)), 68 mW/m(2) (A(PEM)=6.2 cm(2)), and 190 mW/m(2) (A(PEM)=30.6 cm(2)). PEM surface area was shown to limit power output when the surface area of the PEM was smaller than that of the electrodes due to an increase in internal resistance. When the relative cross sections of the PEM, anode, and cathode were scaled according to 2A(Cat)=A(PEM)=2A(An), the maximum power densities of the three different MFCs, based on the surface area of the PEM (A(PEM)=3.5, 6.2, or 30.6 cm(2)), were the same (168 +/-4.53 mW/m(2)). Increasing the ionic strength and using ferricyanide at the cathode also increased power output.