Resonance fluorescence of the OH radical was observed in the mesosphere by the Imaging Spectrometric Observatory (ISO) on ATLAS 1. A preliminary determination of the OH density profile from 70 to 80 km has been made from these observations. This marks the first measurement of ground state OH in the mesosphere since Anderson's [1971a,b> sounding rocket measurements, and the first from space. ISO imaged resonance scattered sunlight in the OH A2&Sgr;-X2&Pgr; (0,0) band during limb scans at tangent heights between 60 and 85 km, at 1.6 km spatial resolution, using an f/3.5 diffraction grating spectrometer with spectral resolution of 0.5 ¿. OH observations were conducted throughout most of the dayside passes during the mission, covering much of the northern hemisphere to 57¿N latitude. Here we report results from an observation at 39¿N, local solar time 13:15, on March 30, 1992; we find OH densities on the order of 8¿106 cm-3 from 70 to 80 km, decreasing rapidly above 80 km. ¿ American Geophysical Union 1993