The quality of effluent from urban wastewater destined for reuse was compared after treatment by a submerged membrane bioreactor (MBR) and a conventional activated sludge system followed by ultrafiltration (CAS-U). The study was carried out at pilot scale, using an MBR module equipped with hollow-fibre ultrafiltration membranes fed with pre-treated raw urban wastewater, and a flat-membrane ultrafiltration module fed with effluent from a conventional activated sludge plant. A comparative analysis was carried out for physicochemical quality parameters (turbidity, suspended solids, colour and COD) and microbiological quality parameters (E. coli, faecal coliforms, total aerobic bacteria and coliphages). A particle distribution analysis (2-125 mu m) was also undertaken. The two systems produced water with similar phsyicochemical characteristics, although the MBR proved more efficient at reducing COD. However, the colour of the MBR effluent was more marked. With regard to microbiological quality, the differences observed between the two systems were due to variations in the periods of chemical cleaning applied to the membranes in order to prevent progressive fouling of the permeate zone. Independently of the frequency of chemical cleaning, the MBR system proved more efficient at retaining coliphages.