Ion concentration measurements for H+ and D+ from the magnetic ion mass spectrometer on Atmosphere Explorer C satellite are used, in conjunction with other atmospheric data, to determine the concentrations of H and D in the nonpolar daytime thermosphere. The ratio of the observed D+ to H+ concentrations has essentially the same height dependence in the 300- to 800-km region as expected for their neutral counterparts, even in the presence of ion temperature gradients and probable large vertical ion fluxes. Rapid charge exchange with atomic oxygen ensures that [D>/[H>≂[D+>/[H+> at the lower altitudes where the derived D to H concentration ratio is a factor of ~6 larger than its sea level value, for an exospheric temperature of 930 ¿K. This relative enhancement of deuterium arises from the fact that hydrogen more readily escapes the earth, and a large vertical gradient in the H concentration relative to its diffusive equilibrium value is necessary to drive this upward flux through the lower thermosphere. If these planetary losses of hydrogen are much greater than those associated with evaporative escape, as is the current view, then correspondingly larger deuterium loss rates are also likely in order that the thermospheric [D>/[H> ratio not increase well above the observed value. The absolute winter daytime concentration of deuterium at 300 km is found to be 210¿50 atoms/cm3.