In a system of charged SDS micelles the value D′ of the self-diffusion coefficient of tagged counterions is less than its value D in the absence of micelles. The ratio D′/D is derived theoretically for a number for SDS and salt concentrations. The micelles are regarded as uniformly charged impenetrable spheres each at the centre of an electrically neutral “cell” bounded by a spherical surface on which appropriate boundary conditions are assumed. As a preliminary to the calculation of D′/D, the Poisson-Boltzmann equation is solved numerically in the spherical “cell” for a number of cell radii and free electrolyte concentrations. Calculated values of D′/D agree well with experimental results and show that the effect can be explained by the action on the moving ions of the electrostatic fields in the diffuse charged layers around the micelles.