To clarify the effect of sludge retention time (SRT) on performance and microbial behaviour in a submerged membrane bioreactor for the treatment of domestic wastewater, four runs of a laboratory scale reactor with hydraulic retention time of 5 h and SRTs of 5, 10, 20 and 40 days, respectively, were conducted. The membrane bioreactor process was capable of achieving over 90% removals both for chemical oxygen demand and ammonia nitrogen, on average, almost independent of SRT. With prolonged SRT, concentrations of suspended solids and volatile suspended solids in the bioreactor increased accordingly, whereas sludge growth kinetic parameters, sludge yield and endogenous decay coefficients, declined slightly and exponentially, respectively. The mean sludge particle sizes at different SRTs were in the range 14.82-48.24 mum. providing a favourable environment for enhancement of mass transport. Characterized by an oxygen consumption rate, sludge specific activities both for organic decomposition and nitrification of ammonia nitrogen varied with SRT. However, volumetric oxygen consumption rates of the membrane bioreactor, representing the whole ability of the process for decomposing pollutants, were enhanced as SRT increased.