Two submerged anoxic/oxic membrane bioreactors (MBRs) were operated in parallel under same conditions but different aeration intensities. The Aeolosoma hemprichi and Tubificidae proliferated in the MBR operated under high aeration intensity but were never observed in the MBR with low aeration intensity. Sludge yield coefficient, sludge physicochemical characteristics and filtration performance in the two MBRs were systematically investigated. Results showed that the presence of aquatic worms decreased the total phosphorus removal efficiency while it had no impacts on chemical oxygen demand and nitrogen removal. High aeration intensity could decrease the floc size and increase dispersed bacteria population, thus providing more feed for the worms. The worm growth led to a reduction of the sludge production, an increase of soluble microbial products, an improvement of sludge settleability and a deterioration of sludge dewaterability. The excess growth of worms resulted in a higher membrane fouling rate in the MBR. (C) 2011 Elsevier B.V. All rights reserved.