We have observed non‐ohmic conductivity inn‐type semi‐insulating GaAs. It has been accompanied by negative photoconductivity and giant magnetoresistance. Potential probe measurements show that the electric field increases from cathode to anode which indicates that electrons are injected at the cathode. Double injection is improbable on the basis of hole lifetimes and the absence of negative resistance regions. The threshold field for non‐ohmic behavior is weakly dependent on sample length. This implies either that the trapping parameters are strongly influenced by the electric field or that there is appreciable impact ionization of trapped, injected charge.