An attempt was made to have a continuous observation of the zonal wind component in the tropical mesosphere and lower thermosphere for 15 days of August 1--15, 1980, with the aid of the UHF radar at Arecibo (18 ¿N). By using the wind data, together with addtional data obtained from meteorological rockets, meteor radar, and Tiros-N satellite, analyses were made of the time-mean zonal wind profile, tides, long-period waves, and short-period gravity-type waves, respectively. There appears a remarkable day-to-day variation of the zonal wind throughout the region between 70 and 95 km with a characteristic time scale of about 5 days, indicating the passage of traveling planetary-scale waves in the summertime tropical middle atmosphere. It was also found that the short-period oscillation with a time scale of a few 10 min appears intermittently in association with the strong vertical shear of the background flow. This oscillation is considered to be the manifestation of the Kelvin-Helmholtz type instability in the lower thermosphere.