Membrane fouling is the major problem that hinders the practical application of membrane bioreactor (MBR) systems. Since the MBR system includes living microorganisms and their metabolites, the fouling mechanism is even more complex than that of conventional membrane separation processes. This paper provides an improved understanding of the influence of filamentous bacteria on membrane fouling in microfiltration of activated sludge wastewater. In order to make clear the membrane fouling mechanism of filamentous bacteria, the physicochemical characteristics of sludge floes, including extracellular polymeric substance (EPS), zeta potential, relative hydrophobicity (RH) and floe morphology were systematically investigated. The results showed that the sludge floes with negligible filamentous bacteria led to severe membrane pore blocking, and the sludge floes with filamentous bacteria created the formation of a non-porous cake layer on the membrane surface. The excess growth of filamentous bacteria resulted in much more release of EPS, lower zeta potential, higher hydrophobicity of sludge floes and more irregularly shaped floes, which did great harm to membrane filtration. The sludge floes with a small quantity of filamentous bacteria had a positive effect on membrane permeation. It is important to control filamentous bacteria concentration in the operation of MBRs. (c) 2005 Elsevier B.V. All rights reserved.