By means of small-angle neutron-scattering (SANS) experiments, the dependence on concentration and temperature of the mean aggregation number [N] of rodlike tetradecyldimethylaminoxide (TDMAO) micelles in D2O was determined for 278 K < T < 370 K and for 2.69 mM/L < c < 118 mM/L. By an analysis via the so-called ladder model, the difference of the chemical potentials for a monomer located in the end cap and one in the cylindrical part of the spherocylindrical micelles, respectively, was extracted. This difference depends on the concentration c of TDMAO and strongly on temperature T.