A class of amphiphilic macromolecules has been synthesized by combining well-defined polystyrene (PS) with poly(propylene imine) dendrimers. Five different generations, from PS-dendr-NH2 up to PS-dendr-(NH2)(32), were prepared in yields of 70 to 90 percent. Dynamic light scattering, conductivity measurements, and transmission electron microscopy show that in aqueous phases, PS-dendr-(NH2)(32) forms spherical micelles, PS-dendr-(NH2)(16) forms micellar rods, and PS-dendr-(NH2)(8) forms vesicular structures. The lower generations of this class of macromolecules show inverted micellar behavior. The observed effect of amphiphile geometry on aggregation behavior is in qualitative agreement with the theory of Israelachvili et al. The amphiphiles presented here are similar in shape but different in size as compared with traditional surfactants, whereas they are similar in size but different in shape as compared with traditional block copolymers.