The influence of hydrodynamic conditions on membrane fouling was investigated. Ten experiments were dsigned and operated over a wide range of suspended solid (SS) concentrations (2-20 gl-1), iltration fluxes (4.5-271m-2h-1) and aeration intensities (10-100m3 m-2 h-1). In each experiment, the liquid cross-flow velocity was measured and the membrane filtration resistance was monitored. A modified model was set up for calculating the cross-flow velocity of the activated sludge over the membrane surface. The increasing rate of filtration resistance was deduced as a power function of the cross-flow velocities, filtration fluxes and SS concentrations, and a critical curve corresponding to a group of critical fluxes and critical aeration intensities was found. The membrane filtration resistance sharply increased at a flux over the critical flux or at an aeration intensity below the critical aeration intensity.