SO was created by electric discharge in a mixture of SO2 and Ar and by UV-laser photolysis of Cl2SO. The two methods are compared. The pure rotational spectrum of the X(3) Sigma(-) electronic ground state was measured in highly excited vibrational states up to v = 17, that of the a(1) Delta excited state up to v = 13, and that of the b(1) Sigma(+) excited state up to v = 11. Measurements up to 900 GHz without harmonic generation were possible thanks to the use of Russian-constructed BWOs. (C) 1997 Academic Press.