A laboratory study was conducted to investigate the production of quorum sensing (QS) molecules by aerobic granules in membrane-partitioned bioreactor. Flow-chamber (FC) tests with Escherichia coil K12 demonstrated that granules induced more attached growth of E. coil cells than activated sludge flocs, leading to more cell adhesion and biofilm formation on the FC cover slide. Using the thin-layer chromatography, N-acy-L-homoserine lactones (AHLs) with acyl chains shorter than 10 carbons were detected in the liquid phase of granular sludge. Organic substances extracted with acidified ethyl acetate from the supernatant of granular sludge promoted the adhesion and growth of E. coil cells on the glass surface. AHL-like signal molecules were apparently produced by granules and might be involved in the formation of granules and the maintenance of granular structures during wastewater treatment. (C) 2012 Elsevier Ltd. All rights reserved.