The use of membrane bioreactors offers a considerable improvement in process stability and effluent quality for small-scale wastewater treatment plants (Q < 8 m^2/d). The treatment performance of four different in-building membrane bioreactors (Busse-MF of Busse GmbH, Germany) was found to be considerably better than the minimum required by German DIN 4261 part 2. Apart from carbon removal, a stable nitrification and even a partial denitrification was observed. The hygiene quality of the treated wastewater is such that a reuse of the water, e.g. for irrigational use, can be recommended. Due to very low sludge production rates it was possible to operate the bioreactor for ca. 450 days without withdrawal of excess sludge. For up to one year this seems to be reasonable when varying MLSS concentrations can be tolerated. For MLSS concentrations of up to ca. 35 g/L, no oxygen limitation due to a decrease in oxygen mass transfer was observed in the Busse-MF. Includes 8 references, tables, figures.