Observations with the SOUSY VHF Radar in the mesopause region during summer 1989 show layers of strong echo intensity and wind speeds increasing with height. Occasionally, quasi-monochromatic internal gravity waves occur in the bottom side of the echo layer and vanish in the height interval around the echo intensity maximum which tends to increase with increasing wave amplitude. The data analysis can be performed using a Boussinesq and a WKB approximation. It shows that the horizontal phase trace velocity of the observed gravity waves is equal to the background wind velocity at the height of the echo intensity maximum. The observations thus represent nonlinear gravity wave-critical level encounters producing strong turbulence in the neutral gas. But the estimated energy dissipation rates are too small to produce neutral gas turbulence at scales equal to half the radar wavelength yielding an additional indication for a yet unidentified mechanism generating small-scale structures in the electron gas. ¿American Geophysical Union 1991