Although ultrafiltration has impressive capabilities for turbidity removal, its inability to remove significant amounts of natural organic matter (NOM) would seem to be a handicap, particularly in cases in which characteristics of ultrafiltration (UF) membrane systems are very attractive. Pretreatment processes consisting of various combinations of coagulation-flocculation and powdered activated carbon (PAC) adsorption were evaluated for use in UF of a lake water. Permeate quality was judged on the basis of the removal of nonpurgeable organic carbon (NPOC), ultraviolet-light-absorbing compounds, turbidity, and trihalomethane (THM) precursors. The development of permeate flux and the flux recovery after backwash were also monitored and modeled with the series resistance model. It was shown that for ultrafiltration to be effective in removing NPOC and THM precursors, pretreatment with PAC is required. Coagulation treatment before UF also results in the removal of more organic matter than by UF alone but appears to be more important in conditioning the filter cake and enhancing flux. Includes 14 references, tables, figures.