To investigate the effects of external resistance on the biofilm formation and electricity generation of microbial fuel cells (MFCs), active biomass, the content of extracellular polymeric substances (EPS) and the morphology and structure of the biofilms developed at 10, 50, 250 and 1000 Omega are characterized. It is demonstrated that the structure of biofilm plays a crucial role in the maximum power density and sustainable current generation of MFCs. The results show that the maximum power density of the MFCs increases from 0.93 +/- 0.02 W m(-2) to 2.61 +/- 0.18 W m(-2) when the external resistance decreases from 1000 to 50 Omega. However, on further decreasing the external resistance to 10 Omega, the maximum power density decreased to 1.25 +/- 0.01 W m(-2) because of a less active biomass and higher EPS content in the biofilm. Additionally, the 10 Omega MFC shows a highest maximum sustainable current of 8.49 +/- 0.19 A m(-2). This result can be attributed to the existence of void spaces beneficial for proton and buffer transport within the anode biofilm, which maintains a suitable microenvironment for electrochemically active microorganisms. (C) 2011 Elsevier B.V. All rights reserved.