A two-day wave disturbance is observed in the mesospheric temperature and water vapor on the basis of new version data from the Upper Atmosphere Research Satellite Microwave Limb Sounder. Using two data segments during the austral summers (January--February of 1992 and 1993) and the asynoptic mapping method, a strong wave signal is identified as having zonal wave number 3 and a westward period of about 2.1 days. The wave amplitudes are located near the core of the summer easterly jet with strongest wave amplitudes (as large as 11 K and 0.35 part per million by volume) near the mesopause. The temperature and water vapor wave strengths are highly correlated in time, but their peaks are almost longitudinally out of phase. Poleward heat flux associated with upward wave energy propagation in the Southern Hemisphere points to baroclinic instability as the cause for the wave appearance. A growing wave signature in water vapor is observed in regions of strong meridional gradient of water vapor. Near the mesopause, wave breaking is suggested as moist polar air is displaced into the much drier subtropics and wave amplitude decays.