Treatment of secondary and tertiary municipal wastewater by membrane filtration has been investigated at the University of California, Davis, California, USA, as described in the following paper. Secondary and tertiary wastewater used during this investigation were produced from activated sludge and media filtration processes, respectively. Ultrafiltration, hollow-fiber, polysulfone membranes were used in this investigation for the treatment of secondary and tertiary municipal wastewater. Membrane operating parameters investigated were cross-flow velocity, with circulation and single pass flow, trans-membrane pressure and backflushing methods. Total solids and particle size distribution of secondary and tertiary wastewater were analyzed and correlated with observed membrane performance. Reduction of total solids, biological oxygen demand (BOD), and chemical oxygen demand (COD) of secondary wastewater was also determined after treatment with membrane and media filtration processes.