Thin (similar to 25nm) zincblende GaN (1 1 1) epilayers have been grown on hexagonal ZrB2 (0 0 0 1) Substrates by plasma-assisted molecular-beam epitaxy at temperatures near 600 degrees C. Layers grown in near-stoichiomectric conditions exhibit high phase purity; no wurtzite inclusions are detected by high-resolution X-ray diffraction or photoluminescence. For growth in the presence of excess Ga the layers are predominantly zineblende phase, but contain some wurtzite inclusions. Reducing the growth temperature to 400 degrees C results in a much-increased fraction of wurtzite inclusions. The growth of zincblende GaN with high phase purity on hexagonal ZrB2 implies the existence of an underlying physical mechanism rather than simply it result of "growth mistakes." A possible surface energy-related mechanism by which zincblende GaN (1 1 1) call be preferentially nucleated on hexagonal substrates is discussed. (c) 2005 Elsevier B.V. All rights reserved.