The surfactant aggregation behavior in aqueous decylammonium chloride solutions for concentrations between 5 and 29 wt % was studied by H-2 nuclear magnetic relaxation dispersion in the 0.5-77-MHz frequency range. Below the second cmc the micelles can be described as polydisperse fairly small aggregates undergoing unhindered Brownian rotation. Above the second cme the micelles are rod shaped and grow with increasing concentration, and as a consequence the tumbling of the micelles eventually becomes restricted by neighboring aggregates, which is demonstrated by the low-frequency relaxation data. The restricted reorientation of the micelles in the concentrated solution has been modeled as rotational diffusion in a hard-walled double cone potential of mean torque.