The formation and evolution of a three-dimensional magnetotail plasmoid is investigated by means of a three-dimensional resistive MHD simulation. In contrast to earlier simulations, the present one takes place in a tail configuration that includes the transition from a closed field line region to an open far-tail region with a distant X line or separator. This is necessary to identify the plasmoid and to study its properties during the separation from Earth. Particular emphasis is placed on the plasma acceleration and the relative importance of pressure forces and Lorentz forces. Two distinct phases of the plasmoid evolution are analyzed, the growth, characterized by accumulation of plasma that has already been accelerated by Lorentz forces outside the plasmoid, and the severance of the plasmoid from Earth, which takes a finite amount of time, propagating from midnight toward the flanks. This phase is characterized by a decrease in plasmoid speed due to the fact that accumulation of plasma of slower speed continues near the flanks and at the leading edge, while the plasmoid becomes severed near midnight. This phase is also characterized by a complicated topological structure of the plasmoid consisting of intermingled flux tubes with different connections. The total force on the plasmoid itself is initially dominated by the pressure gradients and later by the Lorentz forces. These forces, however, do not contribute significantly to the plasmoid motion in comparison to the accumulation of momentum. ¿1991 American Geophysical Union