Analytic theory has yielded contradictory predictions regarding the stabilizing role of electron dynamics on the collisionless tearing instability in a field-reversal region with finite normal magnetic field component Bz. Two-dimensional (x,z) simulations in which both electrons and ions are treated as particles demonstrate that, for the case of thin current sheets (Ri/&lgr;~1, where Ri is the ion Larmor radius based on the lobe field and &lgr; is the current sheet half thickness) and with weakly chaotic electron orbits, the ion tearing mode is indeed stabilized under the mild condition that k&rgr;e≲1, where &rgr;e is the electron Larmor radius in the Bz field. ¿ American Geophysical Union 1994