We have found that, in the absence of threading dislocations in In xGa1-xN/GaN heterostructures, coherent generation of misfit dislocations occurs for x > 0.11 in ∼100-nm-thick epilayers. We focus this report on In0.17Ga0.83N grown on a low-defect-density GaN free-standing substrate (with 1.9% lattice mismatch). A diffraction contrast analysis carried out in the transmission electron microscope showed straight line defects with Burgers vectors 2/3 〈112̄0〉 (i.e., 2a, where a is the hexagonal plane lattice parameter), which extended many micrometers approximately along 〈11̄00〉 directions and with an average lateral spacing of 90nm. Although these defects were complex and mostly sessile, evidence was found that they can dissociate into glissile misfit dislocations with Burgers vectors of 1/3 〈112̄0〉. It is proposed that the defects are generated by a punch-out mechanism involving slip on inclined prismatic planes. The properties of these defects and their role in relieving misfit strains are discussed. © 2006 The Japan Society of Applied Physics.