Electron diffusion as a function of applied field has been studied for CdTe at 300°K with the Monte Carlo technique. The high energy separation between central and satellite valleys makes the limitations on the use of the macroscopic equation of diffusion particularly severe. At fields around the threshold value for negative differential mobility, polar optical scattering produces uneven initial spreading of a delta distribution of electrons. Furthermore, at fields around and above threshold the large spread of electron velocities causes an initial diffusion much greater than that predicted by the macroscopic equation. This latter effect may be very important in small devices as, in particular, in the dynamics of Gunn domains.