The influences of pumping shear on the performance of a crossflow membrane bioreactor (MBR) were investigated. To compare the intensity of pumping shear, two types of pumps (a centrifugal pump and a rotary one) were used in turn to recirculate mixed liquor. Rotary pump system imposed much stronger shear to microbial floc than centrifugal one and resulted in severe floc breakage. Colloidal particles and organics were liberated from microbial floc by shear and caused rapid loss of membrane permeability by the formation of dense cake layers on the surface of membrane. Recirculation of mixed liquor with a rotary pump gradually increased the soluble COD in the bioreactor and deteriorated microbial activity. After 7 days' operation, specific oxygen uptake rate (SOUR) of microorganisms in rotary pump system reduced to 78%, of initial condition. With a centrifugal pump, however, buildup of soluble COD was not observed and change in microbial activity was negligible. Sludge yield in MBR process was lower than that (0.4-0.5 g MLVSS gCOD(-1)) reported in a conventional activated sludge process: 0.3 g MLVSS gCOD(-1) for the centrifugal pump system and 0.2 g MLVSS gCOD(-1) for the rotary pump system.