The expansion of a semi‐infinite, uniform plasma into a vacuum is studied by computer simulation. The solution resembles an analytic similarity solution except for a discontinuous front that precedes the plasma at a velocity of about3(kT /mi)1/2. Thus, previous speculation that the initial burst of ions would be accelerated to velocities comparable to that of the thermal velocity of the plasma electrons on the basis of a fluid model alone is found to benotvalid.