Galileo Energetic Particles Detector (EPD) measurements indicate that convective flow speeds near Ganymede decrease to less than one-third of rigid corotation well before any significant increase in the magnetic field strength along the trajectory is observed. In an Alfv¿n wing model, such a deceleration implies that the Ganymede magnetosphere is a conducting object. We assess the magnitude of various conductivities and find that closure of the Alfv¿n wing current is attainable only through the body of Ganymede itself. We discuss briefly the planetological implications of this magnetospheric result. ¿ 1998 American Geophysical Union