Rocket ultraviolet spectra obtained at twilight with a solar zenith angle of 110¿ show the presence of the forbidden 2P-4S transition of O+ at 2470 ¿. The observations were made with the spectrometer viewing toward the sunset horizon at an angle of 90¿ to the local vertical. At 140 km the observed emission rate is 175¿25 R, but this includes an estimated 55-R contribution from the coincident NO &ggr; (0,2) band. Both direct photo-ionization of atomic oxygen and photoelectron impact ionization are sources of excitation of the O-(2P) state, and the results of a model calculation are in good agreement with the data. Quenching of O+(2P) is ignored in the model, since most of the observed emission is produced above 300 km.