The structure of Mars' atmosphere was mesured in situ by instruments on board the two Viking landers from an altitude of 120 km to near the surface. The two entries were separated by 178¿ in longitude, 25¿ in latitude, 45 days elapsed time, and 6 hours in Mars local time. Atmosphere structure was very well defined by the measurements and was generally similar at the sites. Viking 1 and 2 surface pressures and temperatures were 7.62 and 7.81 mbar and 2.38¿K and 226¿K, respectively, while pressures at the elevation of the reference ellipsoid were 6.74 and 6.30 mbar. Mean temperature decreased with a lapse rate of about 1.6¿K/km, significantly subadiabatic, from above the boundary layer to about 40 km, then was near isothermal but with a large-amplitude wave superimposed, attributed to the diurnal thermal tide. The mean profile appears to be governed by radiative equilibrium. Differences between the two temperature profiles are due to diurnal effects in the boundary layer, a small cooling of the Viking 2 profile up to 40 km due to latitude and season, and effects of time of day, latitude, terrain, and season on the wave structure. The density data merge well with those of the upper-atmosphere mass spectrometer to define a continuous profile to 200 km. The temperature wave continues above 100 km, increasing in amplitude and wavelength.