The pure rotational transitions of N35Cl in the second electronically excited state, b 1Σ+, were detected together with those of the isotopic species N37Cl by microwave spectroscopy. The NCl radical in the b 1Σ+ state was generated by dc-glow discharge of a N2/Cl2 mixture. The rotational constant, centrifugal distortion constant, and quadrupole coupling constants of the Cl nucleus were determined. The observation of rotational transitions in the vibrationally excited v = 1 state made it possible to derive equilibrium structural parameters. A discussion on the internuclear potential of the three low-lying electronic states of NCl is given.