A longitudinal variation in the mesopause semi-annual oscillation (SAO) is documented from wind observations made by the High Resolution Doppler Imager satellite instrument. The asymmetry is dominated by a wavenumber 1 pattern with a variable phase. During the easterly phase of the mesopause SAO (equinoxes), maximum tropical easterlies occur over eastern South America and the Atlantic Ocean in the longitude band 300¿--360¿. During the westerly phase (solstices), the westerly winds descend more rapidly and are larger below ~80 km in this same (Atlantic) longitude band. To the best that can be determined with sparse data, the longitudinal asymmetries near the stratopause are weak and are in phase with those in the mesosphere.¿ 1997 American Geophysical Union