The low-frequency variability in the equatorial middle atmosphere is dominated by the semiannual oscillation (SAO) and quasi-biennial (QBO) oscillation. Although the general characteristics and forcing mechanisms of these oscillations are thought to be known, these oscillations are still not well reproduced in modeling experiments of the middle atmosphere. The relatively long period (1992--1996) of Upper Atmosphere Research Satellite (UARS) measurements and U.K. Meteorological Office (UKMO) assimilated model data have been used to examine the zonal wind SAO in the upper stratosphere. The SAO in zonal wind is found to be latitudinally asymmetric about the equator with strongest amplitudes in the southern hemisphere subtropics. The latitudinal asymmetry is diagnosed by using the TEM momentum equation and UKMO data. The asymmetry is mostly due to the different phasing of the peak advective forcing by the mean meridional circulation in each hemisphere. The contribution of gravity waves to the zonal wind SAO are also explored by using the forcing from a linear gravity wave model. Gravity waves are found to be the primary source of westerly momentum in each hemisphere during fall and contribute to the latitudinal asymmetry of the SAO by having more easterly forcing in the southern hemisphere during summer compared to in the northern hemisphere during summer. ¿ 1998 American Geophysical Union