A combination of transmission electron microscopy imaging and diffraction techniques is used to characterize crystal defects in homoepitaxial GaN thin films. The Burgers vectors of dislocations is established by combining large-angle convergent beam electron diffraction and conventional diffraction contrast techniques. It is shown that dislocations with Burgers vectors c, a, and c+a are present. Evidence is presented that dislocation segments lying in the interfacial plane are dissociated on a fine scale. The significance of the observations for understanding homoepitaxial growth of GaN is discussed. © 1996 American Institute of Physics.