On 6 September 1996 plasma measurements were obtained in the vicinity of Ganymede as the Galileo spacecraft passed by this moon with a closest approach distance of 261 km. Near Ganymede a dense, cold plasma region was found to be embedded in Jupiter's hot plasma sheet. The cold plasmas are hydrogen ions flowing outwards from Ganymede at supersonic speeds. Temperatures and maximum number densities of these ions were about 4¿104 K and 100/cm3, respectively. Over Ganymede's polar caps there is strong plasma convection with speeds in the range of 50 km/s, i.e., the flow is not primarily directed parallel to the local magnetic fields. The corresponding potentials across a Ganymede diameter are in excess of 100 kV. The primary source of the hydrogen ions is believed to be the water ices on the surface of Ganymede. However, the anticipated oxygen ion outflow is not present, which implies that the oxygen is left on the moon's surface. The loss of hydrogen from Ganymede's surface is about 3¿109 grams/year.¿ 1997 American Geophysical Union