Rotational spectra of six isotopomers of the propynonyl radical-HCCCO, DCCCO, (HCCCO)-C-13, (HCCCO)-C-13, (HCCCO)-C-13, and (HCCCO)-O-18-have been measured and analyzed, yielding the geometry and leading hyperfine constants of its (2)A’ vibronic ground state. The radical is found to have an acetylenic carbon chain with the unpaired electron strongly localized on the carbon atom C-c, with the carbon atom positions labeled according to HCaCbCcO. The geometry, assumed to be fully trans, is given by the parameters theta(HCC) = 168 degrees theta(CCC) = 163 degrees, theta(CCO) = 136.5 degrees, r(HCa) = 1.060 Angstrom, r(CaCb) = 1.219 Angstrom, r(CbCc) = 1.387 Angstrom, and r(CcO) = 1.192 Angstrom. The Fermi contact hyperfine constants are a(c)(H) = -11.593(41) MHz, a(c)(C-13(a)) = 35.8(1.4) MHz, a(c)(C-13(b)) = 166.2(3.8) MHz, and a(c)(C-13(c)) = 347.6(3.2) MHz, where dipolar terms in the proton and C-13(a) hyperfine structure have been neglected. No evidence of other HCCCO isomers is found in the spectra, although ab initio calculations identify a second minimum at a cumulenic structure with the unpaired electron localized at atom C-a. Brief investigation of the formation mechanism for HCCCO from C2H2 and CO in our apparatus indicates that roughly half the HCCCO is formed without breaking the CO bond.