The aggregated structure of sodium dodecyl sulfate (SDS) adsorbed to the grapite-solution interface has been determined. Atomic force microscopy reveals that SDS adsorbs in periodic structures when the solution concentration is in the range 2.8-81 mM. Using previously obtained adsorption isotherms, we deduce that these structures are hemicylindrical, but we are not able to determine their length. The long axes of the hemicylinders lie parallel in grains which typically extend over (500 nm)(2) to (1000 nm)(2), but the grain size can be reduced by adsorption of other species from solution. Two basic types of grain boundaries have been identified : broad boundaries, where the periodicity of both grains continue into the boundary for several periods, and narrow boundaries, where one or both of the hemicylindrical arrays terminate within a short distance. The period within each grain decreases when the concentration of SDS or the concentration of added NaCl is increased and approaches the diameter of bulk micelles at high concentration. In NaCl solutions, the period is proportional to the solution Debye length. We propose that this is a result of a decrease in interaggregate spacing rather than a decrease in aggregate size. Using a simple geometric argument, we suggest that the curvature of surfactant aggregates on hydrophobic surfaces will usually be lower than that of aggregates in bulk solution with which they are in equilibrium.