The development of fouling in a cross-flow microfilter coupled to an anaerobic bioreactor has been studied. The membrane bioreactor (MBR) was operated at a fixed flux substantially lower than the nominal 'critical flux' of the feed, measured by flux-stepping. A characteristic two-stage transmembrane pressure (TMP) profile was observed with an initially extended period of slow TMP rise followed by a sudden transition to a rapid TMP rise. This TMP profile was reproducible and depended on the imposed flux. The generic nature of this behaviour was confirmed in the MBR literature. Membrane autopsy revealed significant fouling by extracellular polymeric substances (EPS) and an uneven distribution of EPS and fouling resistance. The initial and gradual TMP rise is believed to be caused by the deposition of this EPS. The sudden rise in TMP is believed to be caused by local flux in some regions of the membrane increasing (to maintain the fixed average flux) and exceeding the critical flux of the dominant foulant (biomass). Strategies to extend the sustainable operating period are discussed.