Basal-plane slip has been observed in InGaNGaN heterostructures containing a high density of threading dislocations. The InGaN epilayers were deposited on a GaN mesa structure grown by metal-organic chemical vapor deposition on (0001) sapphire substrates using conventional methods. The glide process starts at the edge of the mesas, generating misfit dislocation half loops with a Burger vector of 13 〈11 2- 0〉. The misfit dislocation propagation is shown to depend on the crystalline orientation of the mesa edge and results in relaxation of over 80% of the misfit strain. Pinning of the misfit dislocation loops by threading dislocations does not stop them from propagating into the layer. No reaction is observed between the dislocation loops and the threading dislocations. The number of dislocation loops and their propagation distance increase with the indium content of the InGaN epilayers, indicating that the process is driven by the misfit strain. As a result of plastic relaxation, InGaN epilayers are observed to expand outside the edge of the GaN mesa, forming a ledge. These observations suggest possible ways to overcome lattice mismatch with GaN in order to grow high quality InGaN films with high indium concentration. © 2007 American Institute of Physics.