The galilean moons of Jupiter are substantial bodies-three of them are larger than the Earth’s Moon, and one is larger than Mercury-yet little has been known about them until very recently. The discovery of a magnetosphere(1) and magnetic field(2) associated with Ganymede was surprising, and raised the possibility that other galilean satellites,. particularly Callisto (which is the second largest after Ganymede), also might have an internally generated magnetic held. Here we report observations of plasma waves around Callisto, detected during the recent fly-by of the Galileo spacecraft. The nature of the plasma waves indicates that Callisto, unlike Ganymede, does not have a magnetosphere or an internal magnetic held The electron density near Callisto, however, is substantially higher than that in Jupiter’s magnetosphere at this orbital radius, indicating that Callisto is a significant source of locally generated plasma, This plasma most probably comes from a tenuous atmosphere around Callisto, which may be similar to the hydrogen cloud around Ganymede, as the electron densities are somewhat comparable.