Electron and hole mobilities in silicon have been determined in a region in which ionized impurity scattering is predominant. Resistivities were measured by a four-point probe and impurity concentrations were obtained with radioactive tracers or from thermal neutron activation analysis. Measurements were taken with several Group III and Group V impurities up to concentrations of 6×1019 (cm-3) and 6×1018 (cm-3) for n- and p-type silicon, respectively. The conductivity mobility can be calculated from these data by considering the percentage of ionized impurities. The electron mobility approaches 80 cm2/v-sec and the hole mobility 60 cm2/v-sec for the highest impurity concentrations. The comparison with measured Hall mobilities leads to a ratio μHμc which agrees with theory. A comparison with the existing theory of impurity scattering yields better agreement for n-type silicon than for p-type. In the latter the measured mobilities are considerably smaller than the theoretical values.