We present the first results of a high-energy resolution gamma ray experiment in the upper atmosphere. This balloon experiment used a nitrogen-cooled Ge-Li detector, and its purpose was to search for &ggr; ray lines between 400 keV and 8 MeV. Apart from the 511-keV annihilation line, we detected a feature that we attribute to the 6129-keV line from the second excited state of 16O. For this line we found a flux of 7.3¿10-3 ¿4.4¿10-3 photon/cm2 at ceiling altitude.