The effect of dissolved oxygen (DO) level on rnicrofiltration performance and microbial physiology was investigated in membrane-coupled sequencing batch reactors (MSBR). Two bioreactors employing anoxic/oxic (A/O) and only oxic (-O-) phase, respectively were run in parallel to elucidate how DO level does affect membrane filterability and microbial characteristics of sludge. If TMP is considered as an important parameter for the MSBR system the head loss originated from the water-level decline during the suction step should be taken into account. TMP value associated with the water-level reduction was calculated to 2.0 kPa, indicating it should not be ignored in this stud. When the cycles in MSBR were repeated, the A/O phase resulted in faster rise in TMP. corresponding to greater membrane fouling, compared to the -O- phase. Permeate flux as well as airflow rate proved to be a major factor affecting the membrane performance for the A/O phase. Analyses of particle size distribution and specific cake resistance indicates that the floes in the A/O phase, smaller than in the -O- phase, tend to form a dense and compact cake on the membrane surface and give rise to a higher hydraulic resistance. Irreversible membrane fouling also developed faster in the A/O than in the -O-reactor. Analyses of microbial community by FISH and DGGE showed a significant difference in microbial community between two reactors. However, no microbial community difference was found between suspended and attached microorganisms for the same reactor. (c) 2005 Elsevier Ltd. All rights reserved.