Fe(III) supplemented into microbial fuel cells (MFC) at initial start-up increased electricity generation by Shewanella oneidensis MR-1. The maximum power density reached 158.1 mW/m(2) with 6 mM Fe(III), compared with 73.9 mW/m(2) without Fe(III). A significant increase in power output was observed when MFC with established biofilm in the absence of Fe(III) was supplemented with Fe(III) afterward. The cell biomass of the anode biofilm of MFCs operated in the presence of Fe(III) was higher than that obtained without Fe(III). An optimum concentration of Fe(III) obviously improved electrochemical activity and the flavins secretion of strain MR-1. The results demonstrated that Fe(III) addition can enhance power generation by strain MR-1 due to the increase of cell biomass and flavins secretion. (C) 2012 Elsevier Ltd. All rights reserved.