Conductive polyaniline nanofibers (PANInf) have been synthesized by interfacial polymerization and used to prepare composite electrodes with carbon black. Thus prepared PANInf/carbon black composite (PANInf/C) has been employed as an oxygen reduction catalyst in a microbial fuel cell. Higher electrocatalytic activity for the oxygen reduction compared to pristine PANInf was resulted when the PANInf/C composite was used. Large enhancement in power density was observed when this composite cathode was used. The maximum power density of 185 mW m(-2) for the pristine PANInf increased to 496 mW m(-2) for the composite cathode. Although the power density is still lower than that of conventional Pt catalyst (604.3 mW m(-2)), a facile bulk synthesis and cheaper price would make PANInf/C an alternative to Pt when a large scale application comes to an issue. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.049205jes] All rights reserved.