Generally, membrane biofilm have been believed to be minimized during the operation of membrane bioreactor (MBR) for wastewater treatment and reuse. In this study, positive roles of membrane biofilm in the intermittently aerated MBR system were highlighted. During the long-term operation using real domestic wastewater, membrane biofilm played secondary filtration barrier for both low and high molecular weight organic matters. The biofilm on the membrane surface was responsible for the removal of low molecular weight organic matters by the use of easily degradable organic matters (i.e. <1 kDa) as microbial carbon and energy sources during the filtration. Also, track study showed that the significant denitrification took place by the membrane biofilm, proved by the degradation of carbon source and increase of alkalinity such as inorganic carbon. Nevertheless the necessity of further study, this study will give unique insight into positive roles of membrane biofilm, and also continue to aid both fundamental studies and developments of MBR processes. ？ 2006.