Mesospheric short-period wind fluctuations are investigated, using two sets of daytime wind data obtained on 15 consecutive days in August 1980 and March 1981 by the UHF Doppler radar at Arecibo, Puerto Rico (18.4 ¿N, 66.8 ¿W). The wind fluctuations are found to have a conspicuous recurring structure with periods of 10--20 min on a vertical scale less than a few kilometers. This structure, appearing as a ''grill pattern'' in the time-height sections of wind velocity, seems to indicate that both upward and downward propagating waves with similar wave parameters exist simultaneously in this height range. The wave energy is shown to be almost equally distributed between the upward and downward propagating waves. The frequency power spectrum has a falling gradient gentler than that of the f-5/3 and even of the f-1 power law ( f is frequency). Occurrence of these waves seems to be related to strong wind shears associated with long-period internal inertia-gravity waves present in the mesospheric region.