Studies on anaerobic membrane bioreactor (AMBR) show high variability about filterability. Therefore, in this study, a laboratory-scale submerged AMBR (SAMBR) was operated with the aim of identifying operational conditions and sludge characteristics that influence the membrane flux. Short-term experiments applying the flux-step method resulted in values of the critical flux (Jc) variable depending on the operational condition applied in the SAMBR. The application of different parameters for the identification of Jc gave comparable results. Although the plant was operated applying different operational conditions, a rapid membrane fouling was usually observed. The applicable fluxes were between 2 and 5 L/(m(2) h) depending on operational conditions. Therefore, the results confirm the much lower sludge filterability in anaerobic than aerobic MBRs. Multiple regression analysis showed that the supernatant composition plays the most important role in membrane fouling. (C) 2009 Elsevier B.V. All rights reserved.