By means of a small-angle neutron scattering experiment the radius R as well as the scattering length density of rodlike tetradecyldimethylaminoxide (TDMAO) micelles in D2O were determined. The micelles turned out to be homogeneous. As a function of the concentration c of TDMAO we found the mean aggregation number (N), which increases with c, and the value of two thermodynamic parameters describing the energy advantage by inserting a monomer in the end cap of the spherocylindrical micelles and in the cylindrical part, respectively.