Using a forced Rayleigh light-scattering technique, we have measured the mutual diffusion coefficients in a side-chain nematic polymer in the two directions parallel and perpendicular to the director, and observed an inverted anisotropy compared to conventional nematic media. These results are interpreted in terms of the influence of the chain backbone on the nematic medium, as regards both the sign of the anisotropy and the order of magnitude of the diffusion coefficients