Data from the Galileo orbiter suggest that two of Jupiter’s moons, lo and Ganymede, have intrinsic magnetic fields. The magnetic field of Jupiter alters the nature of the magnetohydrodynamic processes generating these intrinsic fields. Such an imposed field allows appreciable internal fields to be generated in cases where convection cannot otherwise maintain a dynamo. The dipole moment of the internal field can then become aligned with the background field, as is observed for lo and Ganymede, to might not have a self-sustained intrinsic field in the absence of the ambient jovian field; Ganymede is almost certainly operating as a dynamo in its own right.