Ozone densities in the upper mesosphere have been measured as a function of time and latitude over a two-year period (1982--1983) by the Solar Mesosphere Explorer (SME) satellite. Large seasonal changes occur, particularly near 80 km where ozone densities at the equinoxes are about 2--3 times those observed at the solstices. The structure of the ozone secondary maximum also fluctuates substantially during the year, with extremes near equinox and solstice. Further, the variations are highly repeatable from year to year and from hemisphere to hemisphere. We propose that the seasonal variability in ozone is produced by the variation of Gravity-wave-induced transport in the mesosphere which, in turn, results from the seasonal modulation of the propagation and breaking of small-scale gravity waves.