A pilot-scale membrane bioreactor was operated for advanced treatment of secondary effluent from a wastewater treatment plant. The performance and biomass characteristics were evaluated under two different hydraulic retention time (HRT) values (7.5 and 15 hours). The system was operated successfully without biomass purge and, after an initial phase, evolved until reaching a constant mixed-liquor volatile suspended solids (MLVSS) concentration, which resulted in the same value of carbon utilization rate (0.067 +/- 0.004 kg COD kg(-1) MLVSS d(-1)) for both HRTs. Respiration tests (maximum and endogenous oxygen uptake rates) confirmed a low microbial activity, which approached constant values (0.32 +/- 0.001 and 0.064 +/- 0.007 g O2 g(-1) MLVSS d(-1), respectively), regardless of the HRT. The sludge morphology and bound and supernatant extracellular polymeric substances also were assessed. Membrane performance was characterized by a stable behavior occurring simultaneously with a high level of biodegradation in the microcolloidal and soluble fraction.