Membrane filtration of biomass is usually accompanied by significant flux decline due to cake-layer formation and fouling. Crossflow filtration with flux controlled by pumping the permeate can produce stable fluxes if a (critical flux) is not exceeded. Below critical flux the transmembrane pressure is typically very low and increases Linearly with imposed flux. Above the critical flux the transmembrane pressure rises rapidly signifying cake-layer formation which is usually accompanied by a continued rise in transmembrane pressure and/or a drop in delivered flux. A range of microfiltration and ultrafiltration membranes with pore sizes from 0.22 to 0.65 mu m and molecular weight cutoff of 100 kDa was used. The feed was an activated sludge mixed Liquor with concentration in the range of 3-10 g dm(-3). The results show that the critical flux depends on feed concentration and crossflow velocity, being higher for higher crossflow velocity or lower feed concentration. Critical flux was also dependent on membrane type, being lower for hydrophobic membranes. Although the transmembrane pressure was higher for the larger pore size membrane, no significant difference in critical flux was observed among different pore size membranes.