The microwave spectrum of CsO has been observed and analyzed, not only in the ground vibrational state, but also in the upsilon = 1-3 excited vibrational states. The CsO radical was generated by the reaction of N2O with Cs vapor, which was produced by the reaction of Li metal with CsCl at 500-530 degrees C. The observed spectra were found to conform to those expected for a (2)Sigma diatomic molecule, thereby establishing the ground electronic state of CsO to be of (2)Sigma. The observed rotational and centrifugal distortion constants yielded the equilibrium bond length and the harmonic vibrational frequency to be 2.300 745 (16) Angstrom and 356.78 (11) cm(-1), respectively, based on the Born-Oppenheimer approximation. A careful examination of the observed spectral pattern definitely concluded that the spin-rotation interaction constant was positive, at variance with the expectation from a simple (2)Sigma/(2)Pi two-states interaction. This observation was interpreted by assuming positive contributions from higher excited electronic states which superseded a negative contribution from the (2)Pi lowest excited state; the latter state was responsible for the large dependence of the spin-rotation interaction constant on the vibrational quantum number and was estimated from this vibrational dependence to be located at 1225 cm(-1) above the ground electronic state. In reverse to the spin-rotation splitting, the hyperfine splitting was found to increase with the vibrational excitation; in the upsilon = 3 state the hyperfine structure was found completely resolved. However, the hyperfine coupling constants did not vary much with the vibrational quantum number, namely the vibrational dependence of the hyperfine splitting was caused primarily by that of the spin-rotation splitting. The observed hyperfine interaction constants indicated that CsO was an ionic molecule.