The retarding potential analyzers on the Viking landers obtained the first in situ measurements of ions from another planetary ionosphere. Mars has an F1 ionosphere layer with a peak ion concentration of approximately 105 cm-3 just below 130-km altitude, of which ~90% are O2+ and 10% CO2+. At higher altitudes, O+ ions were detected with peak concentration near 225 km of less than 103 cm-3. Viking 1 measured ion temperatures of approximately 150¿K near the F1 peak increasing to an apparent exospheric temperature of 210¿K near 175 km. Above this altitude, departures from thermal equilibrium with the neutral gas occur, and T1 increases rapidly to >1000¿K at 250 km. An equatorward horizontal ion velocity of the order of 100--200 m/s was observed near 200 km and near the F1 peak, with a minimum velocity at intermediate heights. Both landers entered the F1 layer at a solar zenith angle near 44¿, though the local times of the Viking 1 and 2 entries were 16:13 and 9:49 LT, respectively. On Viking 2, considerably more structure was observed in the height profiles of ionospheric quantities, although they were similar in shape to the Viking 1 profiles.