The effect of aniline hydrochloride (AHC) on the size and shape of sodium dodecyl sulfate (SDS) micelles has been investigated by dynamic light scattering. A monotonic decrease in the diffusion coefficient of the micelles was observed with an increase in AHC at fixed SDS concentration. This was ascribed to prolate ellipsoidal growth of the micelles due to decrease of the effective headgroup area/molecule by adsorption of AHC on SDS micelles. The length of the micelles can be tuned by controlling the ratio of concentrations of AHC to SDS. Polymerization of aniline in micelles of different sizes leads to the formation of colloidal polyaniline with variable sizes. A direct correlation between size of micelles and size of polyaniline particles was observed. Combination of static and dynamic light scattering experiments reveal that the conformationsof the polymer do not change significantly with size of the colloid.