The role of electron dynamics in collisionless magnetotail reconnection is investigated by using two-dimensional full particle simulations with ion-to-electron mass ratios mi/me set to 1 and 10. The results indicate that the electrons have a stabilizing influence on the ion tearing instability in the sense that growth rates are reduced in runs with the initial electron Larmor radius being sufficiently small in comparison with the ion Larmor radius. A comparison of our results with those obtained by Pritchett [1994> using a significantly different numerical model shows encouraging agreement. The attempt to achieve an approach closer to realistic magnetotail conditions is inhibited by present technical restrictions. ¿ American Geophysical Union 1996