This work reports the main results of a bench scale membrane bioreactor operated for more than 100 days without sludge withdrawal and fed on real municipal wastewater. The experiments were oriented towards three main objectives. Firstly, the performance of the system was evaluated under two different volumetric loading rates (0.8 and 1.7 g CODL(react.)(-1)d(-1)). Secondly, biomass growth and accumulation of solids were assessed and a relationship between sludge concentration and volumetric loading rates was proposed. Thirdly, biomass activity was evaluated through respirometric tests, and endogenous and maximum respiration rates of heterotrophic and nitrifying bacteria were determined. The experimental campaign showed that these systems are easy to manage and very rapid to start-up. The SS concentrations under equilibrium conditions for both experimental periods were slightly lower than 10 times the volumetric loading rates, and the organic loading rates reached the same equilibrium value of 0.12 g CODgTSS(-1)d(-1). Furthermore, under equilibrium conditions the system showed very limited sludge production (0.12 g VSSgCOD(rem)(-1)) and low biomass activity, although it readily responded to load variations. Further work is being carried out to evaluate the performance over the long term.