High‐purityn‐ andp‐type silicon has been irradiated lightly with Co60&ggr; rays. Analysis of the Hall mobility and magnetoresistance data indicate the introduction of levels near the conduction and valence bands. The analysis suggests the presence of triply ionized acceptors and singly ionized donors. The third ionization state of the acceptor apparently produces a level above the center of the band gap. Under more extensive irradiation a double ionization of the donors can account for the observed high resistivity. An initial increase in the magnetic field dependence of the Hall coefficient in bothn‐ andp‐type silicon may be related to the multiple ionization or to radiation annealing. The Hall mobility and transverse magnetoresistance inn‐type gallium arsenide have been studied as a function of impurity concentration and density of defects introduced by fast‐neutron irradiation. The change in the mobility and magnetoresistance with fast‐neutron bombardment suggests the introduction of levels near the band edges and of multiply ionized levels similar to those in silicon.