This study presents the results from the continuous microfiltration of activated sludge with different filamentous bacteria density (expressed as filament index (171)) in a submerged membrane bioreactor (MBR). During the entire experimental duration of 120 days, the filamentous bacteria density increased gradually from FI = 2 to 6 in the beginning, thereafter maintained at a plateau of FI = 6 for about 20 days, and decreased to FI = 4 at the end of the experiment. The relationship between FI and other properties of activated sludge were analysed using statistical analysis. It was found that filamentous bacteria density has no significant effect on bound extracellular polymeric substances (bound EPS, r(p) = -0.343, p = 0.080) and soluble microbial products (SMPs, r(p) = 0.22 1, p = 0.267). However, it has an important impact on floe size (r(p) = 0.944, p = 0.000) and floe structure (r(p) = -0.752, p = 0.000). High FI favored the formation of bigger floes with a loose structure, and vice versa. Further studies on the impact of the activated sludge properties on membrane-fouling rate indicated that variations of the bound EPS may be the major factor contributing to the membrane-fouling rate. Although filamentous bacteria have the ability to change the floe morphology, the effect on the membrane-fouling rate may be negligible. (C) 2007 Elsevier B.V. All rights reserved.