This study focused on how membrane surface hydrophilicity affects fouling in a membrane bioreactor. A lab-scale membrane bioreactor incorporating immersed polytetrafluoroethylene flat-sheet membrane modules with different contact angles (approximately 70 degrees and 130 degrees) was operated using the inocula activated sludge taken from a lab reactor and from a municipal wastewater treatment plant (WWTP). The reactor also contained a polyvinylidene difluoride flat-sheet membrane module as a reference. The hydrophobicity of the WWTP sludge was higher than that of the lab-reactor sludge. Results showed that when lab-reactor sludge was used, increases in transmembrane pressure (TMP) for the hydrophobic membrane with a high contact angle occurred more rapidly than for the hydrophilic membrane with a low contact angle. In contrast, when the WWTP sludge was used. IMP increases for the hydrophilic membrane occurred more rapidly than for the hydrophobic membrane. These results indicate that the effects of surface hydrophilicity on membrane fouling depend on the hydrophobicity of the activated sludge. When the WWTP sludge was used, the hydrophobic membrane exhibited much higher cake layer resistance than did the hydrophilic membrane. Formation of a dense cake layer on hydrophobic polytetrafluoroethylene membranes may deter adhesion of foulants on membrane surfaces. (C) 2014 Elsevier B.V. All rights reserved.