The problem of the linear stability of collisionless tearing modes in the Earth's magnetotail is revisited. It is found that the collisionless tearing mode is linearly unstable with wavelengths of the order of 10 RE. It is shown that an important feature neglected in earlier theories is a nonzero equilibrium By-field. The physics of the instability is elucidated in the context of a simple slab model with Bx≠0, By≠0, Bz=0, and a sheared parabolic model which is representative of the magnetotail in which all three components of the magnetic field are nonzero. The dispersion equation for the instability is obtained by a boundary-layer analysis. The implications of the theory for recent observations on current disruption and diversion during substorms is discussed. ¿ American Geophysical Union 1990