Laboratory-scale colloidal fouling tests, comparing the fouling behavior of cellulose acetate and aromatic polyamide thin-film composite reverse osmosis (RO) membranes, are reported. Fouling of both membranes was studied at identical initial permeation rates so that the effect of the transverse hydrodynamic force (permeation drag) on the fouling of both membranes is comparable. Results showed a significantly higher fouling rate for the thin-film composite membranes compared to that for the cellulose acetate membranes. Addition of an anionic surfactant (sodium dodecyl sulfate, SDS) to mask variations in chemical and electrokinetic surface characteristics of the cellulose acetate and aromatic polyamide membranes resulted in only a small change in the fouling behavior. The higher fouling late for the thin film composite membranes is attributed to surface roughness which is inherent in interfacially polymerized aromatic polyamide composite membranes. AFM and SEM images of the two membrane surfaces strongly support this conclusion. These surface images reveal that the thin-film composite membrane exhibits large-scale surface roughness of ridge-and-valley structure, while the cellulose acetate membrane surface is relatively smooth.