Turbulence is thought to be produced by gravity waves that become unstable in the upper atmosphere. Owing to a relatively short vertical wavelength of the solar diurnal tide, turbulent viscous-type mixing is expected to be particularly important for tidal dissipation in the mesosphere and lower thermosphere (MLT). Within the Doppler-spread parameterization (DSP) of gravity waves, the vertical eddy exchange coefficients may be calculated from the GW energy deposition rate. The DSP, as implemented in the Spectral Mesosphere/Lower Thermosphere Model, is shown to provide realistic simulations of both the diurnal tide and turbulent mixing in the MLT. In particular, the vertical structure and seasonal variations of eddy mixing agree well with in situ observations and independent estimates from global thermal balance. The seasonal variations of turbulence are found to be primarily responsible for a strong semiannual variation of the diurnal tide in agreement with recent satellite observations. ¿ 2001 American Geophysical Union