The seasonal variability of the migrating diurnal tide is studied using a general circulation model (GCM) that extends from the Earth's surface to 97 km. Results from a 2-year simulation reveal a strong semiannual variation in the amplitude of the tide at low latitudes in the mesosphere, in agreement with recent observations from the upper atmosphere research satellite. The northward wind and temperature attain maximum amplitudes at equinox of approximately 60 m/s and 20¿K, respectively, and minima at solstice that are a factor of 2 or more weaker. The reasons for this seasonal variation are explored by diagnosing the migrating diurnal tide solar and deep convective heating in the GCM and by implementing that heating in a linear mechanistic model. The agreement between the two models is quite good in April, but much less so in July when the diurnal tide simulated by the GCM has a much weaker amplitude and longer vertical wavelength in the mesosphere. It is concluded that seasonal variations in the thermal heating in the troposphere and stratosphere cannot account for the seasonal variations in the amplitude of the tidal winds and temperatures in the mesosphere. Although the exact cause of the semiannual amplitude variation has not yet been identified, nonlinear effects are most likely important since a very close correspondence is found between the Eliassen-Palm flux divergence and the tidal amplitudes, with the times and locations of the strongest divergence coinciding with the weakest tidal wind amplitudes. ¿ 1997 American Geophysical Union