A theoretical study of the MgC3 species has been carried out. There are four species lying within 2 kcal/mol and that could be accessible to experimental detection: two open-chain structures, 1s((1)A') and 1s((3)Pi), and two rhombic four-membered rings, 3s((1)A(1)) and 3t((3)A(1)). The rhombic singlet species seems to be the best candidate for the global minimum, though the corresponding triplet is nearly isoenergetic. Our conclusion is in contrast with a previous theoretical study on this system that predicted a three-membered ring species as the global minimum.