Molecular dynamics (MD) simulations have been carried out to investigate the morphology of monolayers of the cationic surfactant cetyltrimethylammonium bromide (C(16)TAB) at the interface between a hydrophobic substrate and an aqueous solution. The simulation system consisted of two flat hydrophobic surfaces separated by a distance of 70 Angstrom along z (direction normal to the interface). Initially, one surface was covered with a uniform monolayer of 94 erect surfactant molecules, with surface area of 45 Angstrom(2)/molecule. The other surface was covered with two hemicylindrical aggregates, each containing 64 surfactant molecules. A 1 ns trajectory was then generated at constant volume and temperature (T = 300 K). The hemicylinder aggregates relaxed to a semielliptical rather than semicircular cross section but were observed to be otherwise stable during the simulation run. The monolayer arrangement was, however, unstable and evolved into hemicylindrical type aggregates. The present simulation provides support for the interpretation of recent atomic force microscopy (AFM) measurements, which claim C(16)TAB forms stable hemicylindrical aggregates at the interface between an aqueous solution and hydrophobic surfaces, such as graphite.