A laboratory anaerobic membrane bioreactors (AnMBRs) (10 L volume) was operated at 30 degrees C and fed with artificial sewage containing 30% protein at COD loading rate 5.1 kg/m(3)-d to investigate membrane fouling with two membranes. Biomass attached to the membrane surface and formed a foulant layer on the membrane. The foulant layers from polyvinylidene fluoride ultrafiltration membranes coated with PEBAX (cPVDF) and an uncoated polyetherimide (PEI) ultrafiltration membranes were analyzed and compared to suspended biomass in the reactor, using terminal restriction fragments (T-RFs) of the 165 rRNA gene and a clone library. One species of OP11 bacteria was present at high relative abundance in the foulant layers of both membranes. By contrast, Bacteroidetes and Firmicutes (LGC) species were present at low relative abundance in the foulant layers but high relative abundance in the suspended biomass. Similar differences were observed for other species. The results suggest that some minority species like OP11 play a direct role in fouling by attaching to the membrane surface while others, including some that likely play a major role in the metabolism of influent organics, play a less important or indirect role. In the AnMBR, the EPS was predominately proteinaceous. EPS and microbial cells of the foulant layer contributed to membrane fouling. The results also indicate that fouling of PEI was faster than cPVDF and this reaffirm the importance of the membrane material in fouling. (C) 2010 Elsevier B.V. All rights reserved.