  We report the observation of heavy-fermion superconducitivity in CeCoIn5 at
Tc =2.3 K. When compared to the pressure-induced Tc of its cubic relative CeIn3
(Tc ~200 mK), the Tc of CeCoIn5 is remarkably high. We suggest that this
difference may arise from magnetically mediated superconductivity in the
layered crystal structure of CeCoIn5 .
