Molecular‐beam‐epitaxial GaN layers change from strongly conductive (ρ≂10−2 Ω cm at 300 K) to semi‐insulating (ρ≂106 Ω cm) as the N flux is increased. Layers grown at low fluxes show strong n‐type conduction, with transport in the conduction band at high temperatures and in a shallow donor band at low temperatures. For layers grown at high N fluxes, the Hall coefficients become too small to measure, suggesting hopping conduction among deep centers. The temperature‐dependent resistivity data are most consistent with multiphonon, rather than single‐phonon, hopping.