The differential diffusion of two species in a turbulent mixing flow is considered. In flows of moderate Reynolds number molecular diffusivities can be a subtantial fraction of the effective turbulent diffusivity and significant differential diffusion can result for species with widely different molecular diffusivities. A theory is presented for the mean and second moments of concentration difference and results are reported for computations for a jet of hydrogen and propane mixing with air. An experiment using Rayleigh scattering to detect the differential diffusion is suggested. Differential diffusion is considered to be very significant in turbulent diffusion flames where molecular and thermal diffusivities are large at high temperatures.