High-resolution spectroscopy of Mars＇ atmosphere with the Hubble Space Telescope revealed the deuterium Lyman alpha line at an intensity of 23 +/- 6 rayleighs. This measured intensity corresponds to HD/H-2 = 1.5 +/- 0.6 x 10(-4), which is smaller by a factor of 11 than HDO/H2O. This indicates that fractionation of HD/H-2 relative to that of HDO/H2O is not kinetically controlled by the rates of formation and destruction of H-2 and HD but is thermodynamically controlled by the isotope exchange HD + H2O <-> HDO + H-2. Molecular hydrogen is strongly depleted in deuterium relative to water on Mars because of the very long lifetime of H-2 (1200 years). The derived isotope fractionation corresponds to an estimate of a planetwide reservoir of water ice about 5 meters thick that is exchangeable with the atmosphere.