Water from Lake Butoniga near the town of Buzet, Croatia, was used as a source for drinking water production. Since lake water has a high concentration of trihalomethane precursors, a treatment was necessary. A process including ozonation, flocculation and filtration was chosen on the basis of preliminary work in a trial pilot plant with a capacity of 10 m(3) h(-1). Although the chosen process succeeded in producing water that met the demands for drinking water, the efficiency of the removal of natural organic matter was relatively low. Ultrafiltration (UF) and nanofiltration (NF) processes were investigated as alternatives and possible upgrades of the process. Experiments were conducted at pilot plants with the Mavibran SP 006A and Romicon PM 10, PM 50, GM 80 and PM 500 UF membranes as well as with the Filmtec NF 45 NF membrane. Since most of the organic matter in the lake water was smaller than 6-8 kD, the use of the NF process was proposed. To avoid fouling of the NF membrane, we used flocculated and filtrated water from the trial plant as NF feed water. This combination produced water of high quality while process parameters remained stable over the entire period of investigation.