The pure rotational spectrum of the CoCN radical has been recorded in the frequency range 350-500 GHz using direct absorption techniques. This study is the first spectroscopic observation of this molecule by any experimental technique. Spectra of Co (CN)-C-13 have been measured as well. These data indicate that this species is linear in its ground electronic state and has the cyanide, as opposed to the isocyanide, geometry. The ground state term has been assigned as (3)Phi(i), based on the measurement of three spin components (Omega=4, 3, and 2) and in analogy to other isovalent cobalt-bearing species. Hyperfine splittings resulting from the Co-59 nuclear spin of I=7/2 were observed in every transition, each of which exhibited an octet pattern. For the lowest energy spin component, Omega=4, vibrational satellite features were also identified arising from the first quantum of the Co-C (v(1)=1) stretch and the v(2)=1 and v(2)=2 quanta of the bending mode, which were split by Renner-Teller interactions. The ground state measurements of CoCN were analyzed with a case a(beta) Hamiltonian, establishing rotational, fine structure, and hyperfine parameters. The vibrational and Co (CN)-C-13 spectra for the Omega=4 component were fit as well. An r(0) structure was also calculated, providing estimates of the Co-C and C-N bond distances, based on the Omega=4 transitions. CoCN is the fourth molecule in the 3d transition metal series to exhibit the linear cyanide structure, along with the Zn, Cu, and Ni analogs. The preference for this geometry, as opposed to the isocyanide form, may indicate a greater degree of covalent bonding in these species. (C) 2004 American Institute of Physics.