Doppler data generated with the Galileo spacecraft’s radio carrier wave were used to measure lo’s external gravitational field. The resulting triaxial field is consistent with the assumption that lo is in tidal and rotational equilibrium. The inescapable conclusion is that it has a large metallic core. If the core is a eutectic mixture of iron and iron sulfide, it comprises 20.2 +/- 7.4 percent of the satellite’s total mass with a radius that is about 52 percent of lo’s mean radius of 1821.3 kilometers; if the core is pure iron, it comprises 10.5 +/- 3.7 percent of the total mass with a radius of about 36 percent of the mean radius.