Numerical computations of the ideal MHD ballooning mode are performed for the 2-D tail plasma sheet model. The computations are done for the analytic model equilibria that include the Earth's 2-D dipole field, as well as for those that do not. It is found that the ballooning mode can be unstable in some class of plasma sheet equilibria where the field lines are not too much stretched but when the plasma beta still exceeds some critical value. The instability is more easily triggered in the equilibria where the Earth's dipole field is fully included. ¿ 1998 American Geophysical Union