Possible causes of a dense network of threading defects in epitaxial hexagonal GaN films grown on various substrates are discussed. We show that these defects originate at the substrate/film interface as the boundaries between differently stacked hexagonal domains, and are created by surface steps on substrates nonisomorphic with wurtzite GaN. We argue that these defects are inherent in the epitaxy of wurtzite films on nonisomorphic substrates. As a result, isomorphic substrates such as ZnO and GaN should be explored for wurtzite nitride growth. Possible effects of these defects on the properties of wurtzite nitrides are briefly discussed. ©1995 American Institute of Physics.