Because a micelle is constituted by rather weak hydrophobic intermolecular interaction, molecular motion in the micelle is quite active and quick. This rapid molecular motion in a long threadlike micelle formed in an aqueous solution of a cationic detergent, cetyltrimethylammonium bromide (CTAB), with sodium salicylate (NaSal), was examined by dielectric relaxation analysis. We found broadly distributed dielectric spectra in the frequency range from 10(6) to 10(9) s(-1) due to molecular motion of micelle-forming molecules, which were proportional in magnitude to the concentration of CTAB. The fastest relaxation mode, around 10(9) s(-1), is attributed to the rotation of anionic pair formed by a headgroup of a cetyltrimethylammonium cation, CTA(+), and a salicylate anion, Sal(-), as: observed in the spherical micelle of CTAB in aqueous solution. On the other hand, a slow relaxation mode, around 10(7) s(-1), is caused by the localized distribution of ions, CTA(+) and Sal(-), in the threadlike micelle after the lifetime of the ionic pair.