Total mass density data from the miniature electrostatic accelerometer (Mesa) experiment on AE-E are analyzed to determine diurnal and semidiurnal tidal variations in the lower thermosphere (150--245 km) at low latitudes (<20¿). The local time density structure changes from predominantly semidiurnal below 180¿5 km to predominantly diurnal above this transition height, with no strong dependence on season. However diurnal phase in summer occur 6 hours (at low heights) to 11/2 hours (at higher heights) earlier than winter, whereas semidiurnal phases during summer occur from 1 hour (at low heights) to 6 hours (at higher heights) later than winter. Analyses of data from all seasons to determine an annual mean semidiurnal amplitude can therefore lead to unrealistically small semidiurnal amplitudes due to phase cancellation effects. These results are compared with analyses of AE-E data by other investigators, and with recent empirical and theoretical models of thermospheric composition and density. Variations between equatorial and middle latitudes as predicted by theory are estimated to be of the order of 20--50% in amplitude and 3--6 hours in phase for the semidiurnal component, 0--20% in amplitude and 0--6 hours in phase for the diurnal component, and 15 km with respect to the transition height from semidiurnal to diurnal predominance. Therefore tidal variations in total mass density as derived from AE-E measurements cannot be extrapolated to middle latitudes without incurring errors of these magnitudes.