Observations of zonally averaged equatorial temperature obtained from the stratospheric and mesospheric sounder (SAMS) aboard Nimbus 7 are presented and compared to similar data from the limb infrared monitor of the stratosphere (LIMS). The SAMS observations demonstrate that a significant seasonal asymmetry exists in the equatorial semiannual oscillation in the sense that the temperature cycle beginning in the northern winter (the ''first'' cycle) is much stronger than the ''second'' cycle beginning six months later. This asymmetry is also seen in the temperature curvature about the equator, implying via thermal wind balance a corresponding asymmetry in the vertical shear of the mean zonal wind. The SAMS data, which provide a true zonal average and extend over several semiannual cycles (1979--1983), therefore confirm the seasonal asymmetry in semiannual wind regimes previously seen in rocketsonde observations near the equator. Two explanations of the asymmetry are offered. First, an improved Kelvin and gravity wave transmissivity in stronger equatorial easterlies implies stronger westerly mean flow acceleration in the first cycle than in the second. Stronger equatorial easterlies, in turn, may result from planetary Rossby wave momentum transport. Second, evidence of strong polar-tropical coupling in the northern winter indicates that mean meridional circulations are present on a global scale. Nonlinear advection by these circulations is quantitatively significant in the semiannual oscillation. ¿ American Geophysical Union 1988