The correlation between extracellular polymeric substances (EPS) and sludge morphology, especially aerobic biogranulation, in membrane bioreactor (MBR) system, was investigated in this study. Two MBR systems were operated in the same nutrient and environmental conditions, into which aerobic floe sludge and granular sludge were respectively inoculated and called as MFSBR and MGSBR. In MFSBR system, aerobic biogranules (0.5 mm in average diameter) were emerged from floc sludge on day 21 of operation, and grew to 1.2 mm on day 31. In MGSBR system, there was a decrease in average diameter of biogranules from 2.8 mm (when inoculated) to 2.1 mm (on day 34 of operation). There was an overgrowth of filamentous bacteria demonstrated by SEM observation. Although polysaccharides were essential, proteins were the predominant component of aerobic granular sludge and the appropriate polysaccharides/proteins ratio was 0.6:1.0 for aerobic granulation in MBR system (under conditions examined). The sludge/supernatant EPS ratio of 44-45 and 110-130 mg/gMLSS of the total EPS amount were favorable for biogranulation. To control the EPS amount in MBR system, weakening the strength of aeration and shortening sludge retention time (SRT) would be beneficial. Even though the development of the aerobic biogranules and stability were still relatively undefined and unknown (not well studied) from this study, it was possible that aerobic biogranules were cultivated from aerobic floc sludge and their morphology was preserved in MBR for long, but not only in sequencing batch reactor (SBR), if the operating parameters were well controlled. (C) 2007 Elsevier B.V. All rights reserved.