A two‐dimensional, three‐fluid code based on the two‐fluid Potter code [MethodsinComputationalPhysics(Academic, New York, 1970), Vol. 9, p. 340] was developed for simulating the plasma focus discharge. With this code it is possible to treat the neutral gas in addition to the plasma components and to model the ionization and recombination phenomena. Thus the sheet dynamics in a plasma focus can be studied and effects investigated such as the occurrence of residual gas (or plasma) density behind the current sheet in the run‐down phase. This is a prerequisite to the occurrence of leak currents, which are one of the causes limiting the performance of large plasma focus devices. It is shown that fast operating foci with small dimensions behave favorably compared with the ‘‘classical’’ Mather focus [MethodsofExperimentalPhysics(Academic, New York, 1971), Vol. 9B, p. 187] with long coaxial electrodes.