The wave-vector dependent, electron initiated, impact ionization transition rates for the cubic and hexagonal phases of ZnS have been numerically determined using a pseudopotential calculated band-structure and wave-vector dependent dielectric function. The rates for both phases show a strong dependence on the relative band structures. A detailed band-by-band analysis of the transition rate in the first Brillouin zone has been performed to determine the wave-vector dependence of the rate. The energy dependent rates have also been computed by averaging the transition rate over energy. It is found that the hexagonal phase of ZnS exhibits a higher ionization transition rate than the cubic phase. A preliminary estimation of the nature of the threshold has been made using a previously published phonon scattering rate.