A six-dimensional potential energy surface (PES) for the singlet electronic ground state of C3H- has been generated by electronic structure calculations using the coupled cluster CCSD(T) approach. Two potential minima are located: the global one, corresponding to an aromatic, cyclic structure ((1)A(1)), and a local one, lying about 0.56 eV higher in energy, with a trans chain structure ((1)A(')). Both minima are found to be separated by a relatively high barrier on the singlet surface but the isomerization process can also proceed via a coupling between the singlet and triplet PESs with a lower barrier. Variational calculations for the vibrational levels (J=0) up to 3000 cm(-1) were carried out for both isomers, taking into account the full dimensionality of the problem. The present results allow a clear distinction between the singlet isomers by infrared experiments. The structure of the vibrational and rotational stacks in both isomers is very different. For some vibrational overtones and combination levels inverse anharmonicity was found. The high density of the vibrational levels particularly in the chain structure suggests the existence of strong anharmonic resonances. In the bent isomer, the excited in-plane nu (5) and out-of-plane nu (6) modes fall into near-degenerate clusters of levels.