Stability of the plasma sheet and plasma sheet boundary layer against the ballooning mode instability is investigated. The equilibrium state of a two-dimensional plasma sheet configuration with parallel sheared flow is modeled. This equilibrium is shown to be ballooning unstable when ΔW is not positive definite, where ΔW is the potential energy. The eigenmode structure of the ballooning mode is found by imposing the boundary conditions that the waves are totally reflected from the ionosphere, and that no waves are coming in from infinity. The eigenmode structure of the unstable ballooning modes is highly oscillatory, extending beyond ~100 RE. The ballooning modes are thus a possible candidate for explaining the MHD waves and other dynamical events observed in the magnetotail by ISEE 3 and other spacecraft. ¿American Geophysical Union 1990