Tests were conducted on a membrane filtration system used as a substitute for conventional sand filtration systems for the removal of turbidities from drinking water. River water was used as raw water. This membrane system consisted of a pre-treatment filter unit using bundles of long fibers, a UF membrane unit, and a posttreatment unit with activated carbon. For about 120 d it was operated successfully in an automatic and continuous mode of operation despite a fluctuation of raw water qualities. This membrane system permitted obtaining better water qualities with lower coagulant dosage levels as compared to a sand filtration system. For instance, the turbidity, total bacteria and coliform bacteria were removed completely. This experimental work was a part of the project, ＇＇The research and development of the membrane filtration system used for water treatment,＇＇ sponsored by the Ministry of Health and Welfare, Japan.