This paper presents the first comparison of simultaneous Viking infrared and ground-based microwave measurements of the Martian atmosphere. The data are examined in order to investigate a 15--20 K difference between microwave and Viking measurements of mid-level (10--40 km) air temperature. These data have been used by Clancy et al. <1990> to suggest that the Martian atmosphere is generally cooler and clearer than observed during the Viking era. This study suggests that the 15--20 K difference, which is most apparent during the non-dust-storm seasons, is not a real temperature difference, but instead results from a disagreement between the measurement techniques. The existence of this instrumental bias implies that the Martian climate has not substantially changed since the Viking era. ¿ 1998 American Geophysical Union