Activated sludge samples of nine different membrane bioreactors were studied to examine their rheological behavior. All sludge samples showed strongly pseudoplastic behavior. Apparent viscosity increased up to factor 100 with decreasing shear rates. MLSS concentration was found to be a major parameter influencing apparent viscosity. Based on the Ostwald equation, an empirical approach for estimating apparent viscosity as a function of MLSS concentration is given. Increasing sludge viscosity is one of the limiting factors for economically reasonable MLSS concentrations in MBR. As an example, the increase in energy consumption with increasing viscosity is shown for a side-stream MBR, taking into account the cross flow velocity along the membrane surface and the oxygen mass transfer. Includes 15 references, table, figures.