A new instability is proposed to explain the observed large-scale structuring of the magnetotail AMPTE barium releases. The stability theory is based on a modified set of MHD equations which allows for large Larmor radius effects, i.e., pi~Rc which is typical of the AMPTE releases (here pi is the ion Larmor radius and Rc is the transverse radius of the cloud). The mode is driven by the deceleration of the cloud and the density gradient at the edge of the cloud. The predicted linear growth rate at eigenmode structure are consistent with AMPTE observations. ¿ American Geophysical Union 1987