BEFORE the recent fly-bys of Ganymede by the Galileo spacecraft, viable models of the internal structure of Jupiter’s largest moon ranged from a uniform mixture of rock and ice to a differential body with a rocky core and any icy mantle(1). Analysis of the Doppler shift of radio signals from Galileo has now shown that Ganymede is strongly differentiated with a relatively dense core surrounded by a thick shell of ice(2). Other instruments have revealed that Ganymede has an intrinsic magnetic field(3,4), which is aligned approximately antiparallel to Jupiter’s magnetic field. Here we argue that these results imply that Ganymede has an outer silicate core surrounding a liquid (or partially liquid) inner core of iron or iron sulphide, and that the magnetic field is generated by dynamo action within this metallic core. Io also appears to have an intrinsic magnetic field(5,6) (antiparallel to that of Jupiter), implying that it too has a metallic core(7) in which the field is generated either by dynamo action or by magneto-convection.