We describe flux tube interchange motion in a corotating magnetosphere, adopting a Hamiltonian formulation that yields a very general criterion for instability. We derive expressions for field-aligned currents that reveal the effect of ionosperic conductivity on interchange motion and we calculate instability growth rates. The absence of net current into the ionosphere demonstrates the bipolar nature of interchange flow patterns. We point out that the convection shielding phenomena in the terrestrial magnetosphere is a direct consequence of the interchange stability of the system. We conclude the paper with an extended analysis of the nature of interchange motion in the Jovian system. We argue that centrifugally driven interchange drives convection and does not give rise to diffusion of Io torus plasma. Neither a large-scale convection nor other forms of unstable interchange overturning appear adequate to explain the plasma distributions detected at Jupiter. ¿ American Geophysical Union 1989