For the purpose of developing a wastewater treatment membrane bioreactor system, cross-flow microfiltration of intermittently aerated activated sludge was carried out, as a part of the solid-liquid separation process in the bioreactor. The tubular alumina microfiltration membrane, having a pore size of 0.5 mu m, was submerged in the activated sludge. A cross-flow stream over the membrane surface was produced by air bubbling, generated by a diffuser situated underneath the membrane. An exponential relationship between the steady-state flux, J(ss), and the causative factors such as operating parameters and fluid characteristics was found and expressed as J(ss) = V-L = K’u(*1.0)MLSS(-0.5), where V-L, K’, u* and MLSS are the Lift velocity, filtration constant, air-liquid two-phase flow velocity and MLSS, respectively. The relationship, as expressed by the equation, is consistent with that for conventional cross-flow filtration using a circulation pump.