Earth-limb observations by the GLO instrument aboard space shuttle mission STS-53 include the first simultaneous resonant-scattering detection of Mg+ (279.6--280.3 nm) and neutral Mg (285.2 nm) in the thermosphere. Although Mg+ emissions are observed in approximately 45% of GLO scans over a wide geographic and local-time range, neutral Mg is observed in fewer than 1% of the measurements, and most of these sightings occur on two consecutive orbits in a near-equatorial patch at local dusk. Neutral Mg is observed at tangent heights between 150 and 230 km, with an instrument-view-column density of 0.4--1.4¿108 cm-2, compared with simultaneous Mg+ values of 1.4--5.8¿109 cm-2. Calculated incloud densities are [Mg+>~250 cm-3 and [Mg>~5 cm-3, and the average ratio of [Mg>+ to [Mg> in the individual measurements is 34:1. ¿ American Geophysical Union 1995