In a series of GaN epilayers grown by metalorganic chemical vapor deposition on sapphire, the GaN rocksalt structure has been detected by x-ray diffraction (XRD) and directly observed by high resolution transmission electron microscopy. The rocksalt GaN phase was found to coexist with zinc-blende GaN and to have a lattice parameter of a=4.074+/-0.004 Angstrom. High stresses, measured by XRD, and microtwins in the samples point to an epitaxial stabilization process. The behavior of the intensity variation of the rocksalt phase XRD reflections with epilayer thickness indicates that the phase is confined near the GaN/sapphire interface, and that phase transformation takes place during high-temperature deposition and not earlier, as might otherwise be expected. (C) 2003 American Institute of Physics.