High-resolution electron microscopy (HREM) was used to investigate the atomic structure of extended defects present inside wurtzite nitride layers. The threading dislocations bear three atomic configurations with 8-atom cycles, 5/7- and 4-atom rings along their line. Inversion domains are bounded by {1010} facets and their boundaries exhibit Ga-Ga and N-N bonds or are reconstructed. The only non-inversion boundaries lie in {1120} lattice planes. A thorough investigation has shown that they are prismatic faults and two atomic configurations have been observed, they have 1/2 (1011) and 1/6(2023) displacement vectors, respectively. (C) 2003 Elsevier Science B.V. All rights reserved.