NF membranes have been drawing much attention in the field of drinking water treatment because they can remove hazardous organic micro-pollutants such as pesticides and THM precursors, as well as hardness. The rejection properties for 10 kinds of aromatic pesticides (0.045-0.194 mg center dot L-1) were investigated with a bench-scale flow cell equipped with a hollow-fiber NF membrane (HNF- 1, the nominal desalting degree 35% at 0.3 MPa). Both the permeate and retentate were returned to the feed tank, and separation experiments were conducted for 10 days. Although the feed and retentate concentrations decreased in the first half of the experimental period, stable rejections were obtained in the latter half of the period. The rejections were calculated based on the average concentrations of the retentate and feed, and the removals were on the initial feed concentration. The removals were in the range of 45.0-93.8%, but the rejections based on the feed concentrations were relatively low (41.0-88.4%). The logarithm of the solute permeability (log B) correlated linearly with the molecular weight of the pesticides. In addition, the batch-type adsorption experiments indicated the following results: all pesticides were adsorbed on the membrane and the adsorption properties were controlled not only by hydrophobicity (log P) of the pesticides but also by molecular planarity.