Radio tracking data from the Viking landers have been analyzed to determine the parameters of the Mars physical ephemeris, the radii of Mars at the landing sites, and the lander locations. The orientation of the Mars rotation axis, referred to the 1950.0 earth mean equator, equinox, and epoch, was determined to be 317.340¿0.003¿ right ascension and 52.710¿0.002¿ declination. The planet's rotation period was determined to be 24 h, 37 min, 22.663¿0.002 s. Analyses indicate that the determination of the motions of the Mars rotation axis will require additional tracking data. The Mars radii at the sites of landers 1 and 2 are 3389.38¿0.06 km and 3381.91¿0.08 km, respectively. The areocentric location of lander 1 is 22.272¿0.002¿N, 47.94¿0.2¿W. The lander 2 location is 47.670¿0.002¿N, 225.71¿0.2¿W. The areocentric right ascensions of the landers are determined to be 277.314¿0.002¿ for lander 1 and 99.546¿0.002¿ for lander 2 at 0000 hours, January 1, 1977 (Julian data 2443144.5). Possible determinations of relativity parameters, solar oblateness, asteroid mass, and variations of the universal gravitational constant, from their effects on the planetary motions, will require the additional tracking data of the Viking extended mission.