Two campaigns from February 1997 to January 1999 have resulted in 18 sets of continuous temperature measurements in the mesopause region over Fort Collins, Colorado (41 ¿N, 105 ¿W), each covering a complete diurnal cycle, reasonably well distributed throughout the year. It is shown that the winter-summer transitions, based on diurnal means, are abrupt and clear, further strengthening the concept of two-level mesopause, and demonstrate the robustness of this picture. Similar to data taken at Urbana, Illinois, our data at Fort Collins also demonstrate the need to observe over a complete diurnal cycle to deduce the uncontaminated mean temperature structure and seasonal variations in the mesopause region. The difference between annual diurnal, nighttime, and daytime means may be ascribed to a temperature wave propagating with a downward phase speed of ~0.8 km/h. Our data show that the nighttime annual mean is colder than the diurnal annual mean by no more than 2 K below 88 km and warmer by no more than 3 K above 88 km where the three means are nearly the same. As in the Urbana data, we also observed the altitude of a secondary temperature minimum in the summer to be ~96 km, which without strong dynamical cooling in the summer would have been the summer mesopause altitude. ¿ 2000 American Geophysical Union