The loss of membrane flux due to fouling is a major impediment to the development of membrane processes for use in drinking water treatment. The objective of this work was to evaluate fouling in nanofiltration (NF) pilot systems fed conventionally-treated (coagulation/sedimentation/filtration) Ohio River water (CT-ORW) with various additional levels of pretreatment. The chosen additional pretreatments were intended to produce waters with varying biological-fouling potential. Five parallel membranes were fed CT-ORW, ozonated CT-ORW, ozonated/biofiltered CT-ORW, CT-ORW reduced to 7 degrees C, and chloraminated CT-ORW. All systems showed significant flux decline indicating that methods beyond those needed for just biogrowth control are required for NF systems treating conventionally-treated surface waters. The NF systems fed ozonated, ozonated/biofiltered, and untreated CT-ORW had the least amount of flux decline over the course of the study; however, they had significant amounts of biological growth. Fouling in these systems was attributed to the deposition of extracellular material (polysaccharides) in the cake layer, either from the biogrowth on the membrane or carryover from the pretreatment. The low-temperature system had greater flux decline, but it had lower biogrowth than the ozonated, and ozonated/biofiltered and untreated CT-ORW systems. Although lower in biogrowth, the deposited organic material in the low-temperature system still showed a strong biological signature (polysaccharides and aminosugars). The chloraminated system had the greatest flux decline, but the least amount of biogrowth. The organic material deposited in the chloraminated system showed a high level of proteinaceous character.