The microwave spectrum of MnO in the (6) Sigma(+) ground electronic state was detected using a source-modulated submillimeter-wave spectrometer. The MnO radical was efficiently generated by de sputtering of manganese flakes placed inside a hollow cathode in the presence of an oxygen and helium mixture. In total, 283 spectral lines were measured in the frequency region of 210-450 GHz for nine rotational transitions, each of which showed six. fine structure line groups consisting of several hyperfine structure components due to the Mn-55 nucleus (l = 5/2). A least-squares analysis of the measured line frequencies resulted in the determination of rotational, fine. and hyperfine coupling constants including higher-order spin-orbit distortion terms for the spin-spin, spin-rotation interactions and the Fermi contact interaction of the Mn nucleus, The hyperfine coupling constants were used to assess plausible molecular orbital bonding models. (C) 1997 American Institute of Physics.