Light scattering of aqueous NaCl solutions of dodecyldimethylammonium chloride has been measured at different surfactant concentrations for different series of NaCl concentrations, and the micelle size and shape have been determined. At low NaCl concentrations the micelle molecular weight is of the order of 10 4 and slightly increases with NaCl concentration. The micelle is spherical or globular. At high NaCl concentrations the micelle molecular weight increases with increasing surfactant concentration and reaches a maximum: at the critical micelle concentration (CMC) small spherical micelles would be formed, but beyond the CMC they associate into large micelles. The molecular weight of large micelles increases sharply with increasing NaCl concentration and exceeds 10 6. The large micelle is rod-like but is flexible. Reduced repulsion among polar head groups induces the transition of micelle shape from sphere to rod. The effect of size of polar head groups on the micelle shape is discussed with reference to the micelle molecular weight of dodecylammonium chloride and dodecyltrimethylammonium chloride.