The start-up of a pilot-scale membrane bioreactor equipped with submerged Ultrafiltration membranes to treat municipal wastewater has been studied. Attention has been paid to determine membrane separation effects on biomass development in a reactor operated without inoculation. Moreover, the activated sludge model no. I has been applied to model biological removal processes. Filtration alone (without biodegradation) removed more than 70% of the influent total COD due to the high particulate COD fraction typically present in municipal wastewaters. Filtration action, retaining bacteria, allowed a rapid increase of the heterotrophic activity permitting to reach efficiencies in COD removal greater than 90% in one to two days. On the other hand, nitrogen removal process needs a few days (five to twenty depending on operational conditions) to develop and stabilise in the reactors because of the required development of the nitrifying biomass. Biomass development was confirmed using respirometric techniques. The activated sludge model no. I with minor modification was capable of simulating reasonably well the biological processes development in the MBR.