In a thin current sheet (&rgr;i0/L≲1, where &rgr;i0 is the ion gyroradius in the lobe field and L is the current sheet half thickness) of the generalized Harris type, the relative ion-electron cross-field drift is comparable to the ion thermal velocity. The three-dimensional stability properties of such a thin current sheet are investigated by means of nonlocal two-fluid theory and two-dimensional and three-dimensional full particle simulations. As was suggested originally by Zhu et al. [1992>, the drift kink mode is found to be of critical importance. For the simple case of no initial Bz field the fluid theory demonstrates that the drift kink mode is a non-MHD mode with a polarization structure such that E1y is an antisymmetric function of z while E1z is a symmetric function with E1z(0)≠0. Two-dimensional (y,z) particle simulations indicate that the nonlinear behavior of this mode is dominated by long-wavelength modes with kyL~1 and frequency &ohgr;&tgr;~&OHgr;i0, where &OHgr;i0 is the ion gyrofrequency in the lobe field. Three-dimensional particle simulations performed on a massively parallel computer show that while the growth rates for the drift kink mode are reduced by the finite Bz, they can still be appreciable (&ggr;/&OHgr;i0≲0.05--0.10). The kyL~1 drift kink modes are always the first to grow in the simulations; subsequently, tearing-like modes with a dominant kx wave vector also become unstable. Implications of these results for the triggering of substorms are discussed. ¿ American Geophysical Union 1996