Hartree-Fock, second-order perturbation theory, configuration interaction including single and double excitations, coupled cluster theory including singles, doubles, and perturbative triples, multiconfiguration self-consistent-field, and density functional calculations have been carried out using large basis sets for the (1)A(1) and B-3(1) electronic states of aziridenium and cyclopropylidene. The aziridenium ion is predicted to be a ground state triplet with a singlet-tripler gap of 10.7 +/- 0.4 kcal/mol; the singlet is a transition state subject to ring opening. Isoelectronic cyclopropylidene is predicted to be a ground state singlet with a singlet-triplet gap of -13.8 +/- 0.3 kcal/mol. The optimized aziridenium triplet geometry manifests a remarkably long carbon-carbon bond (1.745 Angstrom at the MCSCF level); the corresponding bond distance in cyclopropylidene is also long (1.618 Angstrom).