Pure rotational transitions of a carbon-chain free radical, CCCCCl, have been observed by Fourier-transform microwave spectroscopy. The radical was produced in a supersonic free jet by a pulsed discharge in a 0.2%/0.2% mixture of acetylene and carbon tetrachloride diluted in Ne. Transitions with a 21 type spectral pattern were observed in the regions from 11.6 GHz for N = 4-3 to 23.3 GHz for N = 8-7. The hyperfine structures due to the Cl nucleus were clearly resolved, and the rotational, spin-rotation, and hyperfine coupling constants have been precisely determined. (c) 2005 Elsevier B.V. All rights reserved.