Three identical Biosep((R)) membrane bioreactors were operated at different sludge retention times of 8, 15 and 40 d in order to treat municipal wastewater. The design of this process is an aerated activated sludge reactor and a submerged microfiltration hollow-fibre membrane. The objective of this study was to investigate the influence of the sludge retention time on the operation performance of the membrane bioreactors. The operation was separated into two different phases: in phase I the hydraulic retention time was maintained identical in the three membrane bioreactors at 12 h and in phase II the mixed liquor suspended solids concentration was identical at 7-8 g/L. The membrane bioreactors presented very efficient biological wastewater degradation except with an 8 d sludge retention time and 4 h hydraulic retention time. The membrane performance is less dependent on the sludge concentration than the sludge retention time with the laboratory module. The membrane performance showed clearly better results with increasing the sludge retention time from 8 to 40 d. The concentration of the colloidal and soluble polysaccharides of the liquid phase was identified as the predominant parameter causing membrane fouling.