High-level ab initio methods based on the coupled cluster approximation have been used to study properties of several isomers of the C5H radical, a molecule of significant interest in radioastronomy. The three lowest-lying isomers [the well-known linear form (1) as well as two ring-chain structures, HC2C3 (2) and C2C3H (3)] lie within 30 kcal/mol with isomer 2 approximately 5 kcal/mol higher than the lowest-energy isomer 1. The computed rotational constant for the linear isomer is within 0.7% of the value determined in previous experimental analyses. Transition states formed via simple ring-opening mechanisms for the interconversion of the linear and ring-chain isomers have also been located; these lie ca. 27 and 31 kcal/mol above isomers 2 and 3, respectively, indicating reasonable kinetic stability of these structures to isomerization. The computed rotational constants for these isomers should be useful for laboratory and astronomical observation of these species. In addition, four other minimum-energy structures an found to lie somewhat higher in energy. These isomers involve both three- and four-membered carbon rings, as well as a bent-chain structure.