A method for the growth of GaN bulk crystals under ammonothermal conditions is described. Gallium nitride is shown to have a retrograde solubility in ammonobasic solutions. Using polycrystalline GaN as nutrient and hydride vapor phase epitaxy GaN templates as seeds, the crystals were grown in the hot zone. The ammonothermal growth experiments were carried out in ammonobasic solutions in high nickel content autoclaves for up to 3 weeks. Growth rates up to 50 mu m/day were achieved and single crystals of GaN up to 10 x 10 x 1 mm(3) were obtained. The ammonothermal crystals are of high quality, as characterized by optical microscopy, scanning electron microscopy, high-resolution X-ray diffraction and photo luminescence measurements. (c) 2005 Elsevier B.V. All rights reserved.