A 12-hour oscillation in the horizontal motion of the neutral mesosphere near the South Pole has been determined from optical measurements of the Doppler shift of the OH emission in this region of the upper atmosphere. The measurement of this wave's phase progression with longitude shows this wave to be a westward-traveling zonal wavenumber one mode. The absence of significant oscillation at this periodicity in the simultaneously measured mesospheric temperature is consistent with the zonal wavenumber one determination. Tentative assignment of this oscillation to an inertio-gravity wave has been made based on these findings. The observation of this 12-hour periodicity oscillation as a zonal wavenumber one motion precludes its identification as a zonal wavenumber two semidiurnal tide.