Radio tracking data associated with the February 1977 encounters between the Martian satellite Phobos and the Viking Orbiter I space-craft have been analyzed to determine the gravitational constant (GM) of Phobos. A linear error analysis was conducted to determine the selection of data and the parameter solution set which would yield the best estimate of GM. This error analysis indicated that the optimal data set was a data arc beginning just prior to the closest encounter and spanning three consecutive space-craft orbits; data near periapsis were deleted. The most feasible parameter solution set consisted of the Phobos GM and the spacecraft initial conditions. The result of the data analysis was an estimate of (7.3¿0.7) ¿10-4 km3/sec2 for GM of Phobos.