We simulate in two dimensions the expansion of a plasma between biased plane parallel electrodes using the particle‐in‐cell method. Such a plasma is frequently created in many experiments by the interaction of a pulsed laser with atomic vapor or gas stream. We describe the motion of the electrons and ions and reproduce the experimentally observed bulk drift of the plasma toward the high potential electrode. This is explained in terms of the retrograde movement of the ion sheath boundary on one side of the plasma accompanied by ambipolar diffusion on the other. We estimate the reduced ion density in the plasma by observing oscillations in the space‐charge‐limited current. By calculating the plasma decay time constant and ambipolar diffusion coefficient, we note that computer simulation can generate data which is difficult to measure experimentally, and not possible to calculate analytically. ©1995 American Institute of Physics.