New measurements of the pure rotational spectrum of the CoO radical in its X(4)Delta(i) state (v = 0 and v = 1) have been conducted using millimeter-wave direct absorption techniques. CoO was created by the reaction of cobalt vapor and N2O. Spectra arising from the ohm = 3/2 and 1/2 spin levels, both which exhibit Lambda-doubling, have been recorded, as well as from the ohm = 7/2 and 5/2 ladders, which have been studied in the past. Rotational, fine structure, Lambda-doubling and hyperfine parameters for the v = 0 and v = 1 states have been established from a global analysis of all four spin components. The data suggest a near-by (4)Sigma state. (c) 2005 Elsevier B.V. All rights reserved.