This study evaluated the impact of membrane characteristics on membrane fouling in a submerged membrane bioreactor (MBR) treating municipal wastewater. Three types of microfiltration membranes with similar pore size of 0.1 mu m but different materials and pore microstructures -polyester (PETE), polycarbonate (PCTE) and polytetrafluoroethylene (PTFE) - were used. For track-etched membranes, the PETE membrane, which has the lowest pure water permeability (PWP), exhibited more rapid increase of filtration resistance than the PCTE membrane, which has the intermediate PWP, indicating the significance of membrane porosity in MBR fouling. The PTFE membrane, which has interwoven sponge-like microstructure and highest PWP, showed more rapid resistance increase compared to the PCTE track-etched membrane. This was likely due to the rougher surface of the PTFE membrane, which enhanced foulant adhesion. The trends of the filtration resistance of all the membranes gradually appeared similar after they were cleaned physically and chemically, suggesting that chemical cleaning affected MBR fouling in subsequent operation. Sludge characteristics, including sludge concentration, surface charge and hydrophobicity, did not show direct relationship with fouling rate in this study. Surface charge of the PETE membrane and its foulants layer could contribute to a lower zeta potential of the membrane permeate. It was not clearly shown that membrane hydrophobicity was a significant factor in the membrane fouling during a long-term MBR operation. Gradual increase of protein/carbohydrate ratio in bound extracellular polymeric substances was likely to contribute to the increasing fouling propensity of the membranes tested after membrane cleaning. (C) 2008 Elsevier B.V. All rights reserved.