The objective of this study was to quantify the assimilable organic carbon (AOC) in processing water at several stages of a full-scale nanofiltration (NF) water treatment plant. The NF membrane plant investigated was a 45,400 m3/day (12 mgd) water softening facility at Plantation City in southern Florida. The average AOC concentration of raw feed water was estimated at 158 ug/L acetate-C. After pretreatment (acid and antiscalant addition), AOC levels increased by 12.7%, suggesting that pretreatment chemicals used to control scaling may enhance biofouling potential. The results also demonstrated that nanofiltration was capable of effectively removing 63.4% of AOC. A decrease in membrane productivity over time (declined linearly at a rate of approximately 1.3¿10-4 lmh/kPa per day) could be attributed primarily to biological fouling, which was evidenced by considerable AOC consumption (1.17 kg acetate-C per day) in the membrane system. Includes 23 references, tables, figures.