We have found the dominance of a monochromatic inertia gravity wave with a vertical wavelength of approximately 5.6 km from middle and upper atmosphere radar observations of the mesosphere on February 8, 1985. By assuming linear dispersion and polarization equations for gravity waves, we have estimated the intrinsic period and propagation direction of the wave as 8 hours and northward, respectively. The minimum Richardson number for the atmosphere as modifed by the inertia gravity wave was slightly negative near 72 km, which implies that the wave was saturated and generated turbulence through shear and convective instabilities. We observed an echo power enhancement near the altitude of the minimum Richardson number. At 68--78 km altitude we found intermittent fluctuations in radial wind velocities with a period of 9 min. Their phases rapidly reversed near 73 km, where their amplitudes reached a minimum and the Richardson number was less than 1. These fluctuations seem to be attributable to the shear or convective instabilities generated by the saturating gravity wave. ¿ American Geophysical Union 1987