Long-term microbial behaviour was investigated in a membrane bioreactor with complete sludge retention. The total organic carbon concentration in the supernatant slightly increased with time, as attributed to the accumulation of soluble microbial products, especially those macromolecules with molecular weight > 100,000. Accumulation of inorganic components was also observed in the reactor, with MLVSS/MLSS decreasing from 88% to 81.8%. A pseudo-steady state was evident after 200 days of operation when the sludge concentration leveled off at around 16 kg-MLSS m(-3). The corresponding F/M ratio was 0.1 kg COD kg (VSS d)(-1) and the observed sludge yield Y-b was below 0.11 kg VSS kg COD-1. Kinetic analysis revealed a theoretical sludge yield Y of 0.288 kg VSS kg COD-1 and a decay rate b of 0.023 d(-1), respectively, 72% and 46% of the lower limits of those in a conventional activated sludge process. Microbial viability indicated by heterotrophic bacteria amount per MLVSS greatly decreased during the first 200 days, but organic removal performance always remained stable. Membrane permeability, however, decreased with the time owing to the change in the composition of the mixed liquor. (c) 2005 Elsevier Ltd. All rights reserved.