The microstructure of gas source molecular beam epitaxy GaN films deposited on (0001) sapphire is studied by transmission electron microscopy. For a nucleation layer deposited at 500 degrees C, high-quality materials, with only dislocations (density = 5 x 10(9) cm(-2)) in the volume of the film, are obtained. For a nucleation layer deposited at 550 degrees C, the resulting structural quality is poor. Inversion Domains and {11 (2) over bar 0} prismatic defects are observed. (C) 1999 Elsevier Science B.V. All rights reserved.