Membrane bioreactor (MBR) is an important solid-liquid separation technology employed widely in the wastewater treatment. However, membrane fouling is the major problem that hinders the practical application of membrane bioreactor (MBR) systems. A major obstacle for the application of MBRs is the rapid decline of the permeation flux as a result of sludge filamentous bulking. In this paper a series of membrane filtration tests were performed to evaluate the fouling behaviors of bulking sludge and normal sludge. In order to specify the fouling mechanism of bulking sludge, the floe morphology of bulking sludge and normal sludge was studied based on fractal theory and image analysis. The sludge characteristics was also evaluated in terms of extracellular polymeric substances (EPS), relative hydrophobicity (RH) and sludge viscosity. The results showed that the bulking sludge resulted in a greater flux decline rate than normal sludge. The bulking sludge could cause a severe cake fouling, which was induced by the fixing action of filamentous bacteria. The results also showed that the sludge flocs had a good fractal characteristic, and the boundary fractal dimension of bulking sludge flocs was much larger than that of normal sludge. The flocs of bulking sludge had more irregular shape than the flocs of normal sludge. The irregular shape of bulking sludge would do great harm to membrane filtration process. The higher EPS concentration, RH and sludge viscosity of bulking sludge would speed up membrane fouling further. (c) 2006 Elsevier B.V. All rights reserved.