We observe in Si-doped GaAs, by capacitance transient spectroscopy, electronic occupation of a highly localized state of the donor-related DX center. The emission and capture kinetics are those of a metastable state which lies above the conduction-band edge. The state is so spatially localized that its emission kinetics are not measurably perturbed by neighboring Si atoms (donors or acceptors) at an average distance 3.5 nm. Occupation of this state is therefore a previously unsuspected mechanism which can limit the free-carrier density in very heavily doped n-GaAs. © 1988 The American Physical Society.