We report on observations of small-scale gravity wave breaking into turbulence around the mesopause height using an OH airglow CCD camera and a meteor radar performed on December 23, 1995 in Japan. The small-scale gravity wave propagated against the background wind with a horizontal wavelength of ~27 km, an apparent horizontal phase velocity of ~7 ms-1, and an intrinsic period of ~5.6 minutes. Wave breaking event began with an appearance of smaller-scale structures, which were aligned parallel to the phase fronts of the breaking wave and advanced into more random, turbulence-like features. The instability structures were present in the OH images for about 40 minutes. ¿ 2001 American Geophysical Union