Urban effluents were treated in a pilot membrane bioreactor (MBR) fed by raw wastewater from a sewage plant. The MBR consisted of an aerated tank fed with raw water containing a volume of 600 l of activated sludge which was recirculated through an external circuit containing a ceramic membrane of 0.25 m(2) area with 0.1 mu m pores. Suspended solids (SS) concentration was stabilized at 10 g/l, The sludge and hydraulic residence times (SRT and HRT) were 60 days and 24 h, respectively. Elimination of chemical oxygen demand was higher than 95%. Rheological tests showed that activated sludges from the MBR and from the aeration tank of a classical plant were non-Newtonian. In order to evaluate the contribution of various sludge fractions to fouling, SS were first separated from the sludge by settling, then colloids were separated from the supernatant by a combination of flocculation and settling. The relative contributions of SS, colloids, and dissolved molecules (DM) to the resistance to filtration caused by fouling were found to be 65%, 30% and 5%, respectively. The filtration resistance due to each constituent was also measured as a function of its concentration. The permeate flux did not decrease much when cellular concentration rose from 2 to 6 g/l, The measured total filtration resistance was then compared with the sum of resistances of each constituents at the same respective concentrations as in the sludge. This calculated sum was found to be 50% higher than the measured total resistance, indicating that fouling resistances caused by each constituent were not additive.