The latitudinal and seasonal variations of the CO2 concentration between 70 and 100 km are investigated using the 4.6 μm daytime measurements made by the Improved Stratospheric and Mesospheric Sounder (ISAMS) on the Upper Atmospheric Research Satellite (UARS). The analysis uses a detailed nonlocal thermodynamic equilibrium model of the CO2 states emitting in the 4.3 μm spectral region and the GENLN2 line-by-line radiance code to update an earlier study by L¿pez-Puertas et al., <1998b>. The differences in the volume mixing ratio between the equator and the illuminated pole in the upper mesosphere are shown to be smaller than the detection limit established by the uncertainties in the data (approximately 10% at 85 km, 14% at 90 km, 23% at 95 km, and 28% at 100 km). ¿ 2000 American Geophysical Union