Mesospheric wind measurements made by the MU radar at Shigaraki (34.9¿N, 136.1¿E), Japan, on September 20, 1985, allow us to study gravity wave breaking. The wind data has shown that a monochromatic inertia-gravity wave was propagating vertically, indicating an exponential growth up to a mesospheric level with height. Above that level, however, the amplitude rather decreased. Moreover, the wave-form broke remarkably at a level. In this paper, we show that the breaking of the monochromatic wave can be connected with the convective instability in the wave field, estimating the local stability from the wind data. ¿ American Geophysical Union 1988