This study tested domestic wastewater treatment membrane filtration without external cleaning in sustained long term operation. Domestic wastewater treatment plant monitoring was performed at the municipal wastewater treatment plant Devinska Nova Ves. Bratislava between February 2005 and July 2007. Two membrane modules were tested by immersion in the domestic wastewater treatment plant. The flat sheet membrane module was operated without external cleaning at a flux of 20-60 L/m(2) h for 6 months. The hollow fiber membrane module was operated for 4 Months Without external cleaning with a flux of 20-45 L/m(2)h. Parallel operation of flat sheet and hollow fiber membrane modules showed similar results in effluent water quality. Both membrane modules were able to effectively remove organic matter (as much as 91%) and more than 97% of NH4+-N. Nitrogen removal via denitrification was observed during the short periods with low oxygen concentration. Treated water contained suspended solids under measurable limits.