The dominant mass 2 ion in the ionosphere of Venus is identified as D+ through analysis of the height variation of [mass 2 ion>/[H+> measured in the chemical equilibrium region by the ion mass spectrometer on the Pioneer Venus Orbiter. This result leads to [D>/[H>=(2.2≠0.6) ¿10-2 at the turbopause, which agrees with the ratio measured in the lower atmosphere by the large probe mass spectrometer. The 100-fold deuterium enrichment supports previous suggestions that Venus has lost at least 0.3 percent of a terrestrial ocean.