This paper presents an analysis of a gravity wave event near the tropopause as seen by the vertically looking of the Poker Flat, Alaska, MST radar. The observations are presented as time series of vertical velocity, signal power and spectral width. These parameters are found to oscillate in a systematic fashion that illustrates the effect of gravity waves on the specular echoes. This is seen by most clearly by examining the changing appearance of the Doppler spectrum with the phase of the gravity wave. The principal result is that specular echoes are observed only at certain phases of the wave. Examination of a large sample of data shows that more generally the occurrence of gravity waves tends to destroy specularity. The decrease in the magnitude of specular echoes can be parameterized by the width of their Doppler spectra.