Dynamic light scattering measurements have been performed on micellar solutions of undecylammonium chloride in the presence of 0 to 0.5 mol dm(-3) n-butanol and 0 to 0.2 mol dm(-3) sodium chloride. The results support opposing effects of these additives on micellar growth, with the n-butanol causing a reduction of micelle size, The dependence of the aggregation number on the hydrodynamic radius suggests a transition from sphere to prolate ellipsoid when the aggregation number approximates that predicted for the maximum aggregation number of a spherical micelle, Micelle dimensions calculated from the aggregation number, conformational considerations, and packing criteria of the monomers in the micelle are compared with those from dynamic light scattering data.