We have used a variety of satellite, ground-based, and in situ observations to construct a climatology of the semiannual oscillation (SAO) of the tropical middle atmosphere. The sources of data include rocketsonde observations of winds and temperature, MF radar wind observations, and observations of winds and temperatures made from space by the High Resolution Doppler Imager (HRDI) and the Solar Mesosphere Explorer (SME). These data sets provide a generally consistent picture of the SAO, of the relationship between its stratospheric and mesospheric manifestations, and of its apparent modulation by the stratospheric quasi-biennial oscillation (QBO). In agreement with earlier studies, we find that the first cycle of the stratospheric SAO (which begins with the stratopause easterly phase in northern winter) is stronger than the second cycle (beginning with the easterly phase in southern winter). Similar behavior is apparent in the mesosphere, where the easterly phase is stronger during the first cycle than during the second cycle. HRDI and MF radar are capable of observing the seasonal cycle well into the lower thermosphere. Data from these two sources indicate that a strong SAO is present up to about 90 km, giving way above this altitude to time mean easterly winds with a weak semiannual variation. Between 105 and 110 km, HRDI data indicate the presence of a westerly wind layer with almost no seasonal variation. Apparent modulation of the stratospheric SAO by the QBO is found in rocketsonde data, while HRDI and MF radar observations suggest a correlation between the QBO and the easterly phase of the mesospheric SAO. We discuss the implications of these observations for the wave processes that drive the SAO. ¿ 1997 American Geophysical Union