A nonequilibrium molecular dynamics study of self diffusion in a Lennard-Jones-like fluid shows that the high mass-flux stages of mixing are described well. by generalized hydrodynamics. There is no indication of the damped-wave propagation implied by the Telegrapher＇s equation, and the mass flux is less than that predicted by the diffusion equation. (C) 1998 Elsevier Science B.V. All rights reserved.