Laboratory scale submerged membrane activated sludge process (SMASP) units were operated at various sludge retention times (SRTs) (9.6-498 d) and BOD loadings (0.25-1.3 kg/(m(3).-d)). Under these conditions, the quantity of excess sludge production and the process stability was studied, using synthetic wastewater. The stabilized MLSS was increased with an increase of SRT and BOD loading. The excess sludge production was decreased with an increase of SRT and decrease of BOD loading. The excess sludge production in SMASP at long SRT and low BOD loading was significantly smaller than the conventional activated sludge process. Various test conditions proved that the long SRT and low BOD loading were suitable operational conditions for long stable operation of SMASP.