Observations of medium-scale acoustic gravity waves in the thermosphere, using multiple HF radars, are presented. Examples of waves from a magnetically quiet interval and a more magnetically active interval are examined along with statistics of gravity wave occurrence. It is shown that during quiet intervals gravity wave phase fronts may extend longitudinally for more than 3400 km. During the more active interval, the level of wave activity is elevated to the point that it is difficult to distinguish individual wave packets. Also, power spectra from the active period showed evidence of a harmonic sequence, indicating the possibility of some nonlinear process acting as the gravity waves propagate. Statistics of occurrence showed that high-latitude medium-scale gravity waves are more likely in the winter than in the summer and that the probability of observing waves on multiple radars is essentially the same as observing waves on any one radar. ¿ American Geophysical Union 1996