The structure of short period (2--4 days) planetary waves in the Antarctic mesosphere has been determined from ground based wind measurements. At the South Pole observations are made with a Fabry--Perot spectrometer (FPS) to determine the meridional wind component of OH tracer molecules. Azimuth scanning of the FPS shows that the observed modes are of zonal wavenumber one, as expected at the rotation pole in the neutral atmosphere. At Scott Base (78¿S) observations are made with an MF spaced-antenna mode mesospheric wind-profiler radar. The vertical structure of the waves at altitudes of 80--100 km shows a phase variation corresponding to a vertical wavelength of more than 100 km. The observed periods support previous interpretations, based on satellite observations and barotropic instability models, of modes associated with the '4-day' wave. ¿ American Geophysical Union 1993