Beginning in 1991, Na lidars were used to make routine measurements of the temperature structure between 80 and 105 km altitude during 65 nights at Urbana, IL and 116 nights at Ft. Collins, CO. More than 13,000 temperature profiles representing more than 1100 h of observations were used to characterize the seasonal temperature variations in the upper mesosphere and lower thermosphere. The seasonal behavior is quite similar at the two sites. The mesopause altitude is near 100 km in winter during Nov-Feb and near 86 km in summer during May-Jul. The mesopause temperature varies from 190 K in mid-winter to 175 K in mid-summer. The transitions beween winter and summer mesopause structures occur rapidly in Mar-Apr and Aug-Sep. The temperature differences between the two sites are less than ¿15 K and appear to be the result of atmospheric planetary, tidal and gravity wave perturbations. ¿ American Geophysical Union 1994