The structure of surfactant surface-aggregates can be controlled by mixing two surfactants which, individually, form different surface-aggregate structures. Using atomic force microscopy (AFM), we have found that mixtures of (dodecyldimethylammonio)propanesulfonate (DDAPS) and dodecyltrimethylammonium bromide (DTAB) at the interface between mica and aqueous solution form aggregates ("hemimicelles") which have a length dictated by the proportion of each surfactant in the mixture. In pure DDAPS, the two axes of the aggregate parallel to the mica-solution interface are similar in length and similar to the diameter of a bulk micelle. As the mole fraction of DTAB in solution is increased, one axis of the surface aggregates increases while the other remains almost constant. In the limit of pure DTAB solutions, the aspect ratio is large (> 10). The length of the mixed aggregates is not a linear function of the mole fraction of DTAB in bulk, but this car, be partially rationalized on the basis of electrostatic interactions which concentrate DTA(+) at the mica-solution interface at low bulk concentration. Both DTAB and DDAPS form approximately spherical aggregates in bulk solution, so the surface shape transformation cannot be accompanied by an analogous bulk transformation.