The rotational spectral lines of the cyclic C3H radical in the B-2(2) ground electronic state are observed by microwave spectroscopy. The radical is produced in an absorption cell by discharging a mixture of C2H2, CO, and He. The spectral lines of two C-13 isotopic species are also observed. The radical is a planar molecule with C-2 upsilon symmetry, and the r(s) structure is determined as follows : r(s)(C-H)=1.0760 Angstrom A, r(s)(C-C)=1.3771 Angstrom A, and r(s)(C-CH)=1.3739 Angstrom A. From the observed inertial defect, the vibrational frequency of the C-C antisymmetric vibration is estimated to be about 500 cm(-1), which is fairly low for the frequency of the C-C stretching mode. The low vibrational frequency arises from the vibronic interaction between the ground electronic state and the low-lying (2)A(1) electronic state. On the basis of the hyperfine coupling constants obtained for the C-13 species, the unpaired electron is found to be almost evenly distributed among the three carbon atoms. Such delocalization of the unpaired electron is consistent with the observed geometrical structure.