Low power density, a main drawback in a mediator-type microbial fuel cell (MFC), has been greatly increased by immobilizing bacteria using carbon nanoparticles. Proteus vulgaris-containing carbon paste was prepared by mixing bacteria suspension with carbon nanoparticles using Teflon emulsion. Thus, prepared carbon paste was spread on the carbon cloth and used as an anode. Higher power density was obtained than when bacteria were Suspended in solution. The maximum power density of 705 mW m(-2) was achieved. Various electrode preparation conditions as well as operating conditions have been examined. Impedance measurements suggested that performance improvement came from the decrease in ohmic, charge-transfer, and diffusion resistances. This work demonstrates the importance of bacteria immobilization in enhancing the power density of a MFC. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3190477] All rights reserved.