A number of reports have suggested that InGaAs/GaAs (111)B strained layer epitaxy has the prospect of reaching a higher critical layer thickness than that which can be achieved for (001) substrates. This has motivated a study of the relaxation mechanism of InGaAs/GaAs (111)B quantum wells with high In content (0.12<x<0.35). Transmission electron microscopy has revealed the existence of a different misfit dislocation (MD) configuration for high In contents (x>0.25), which, we believe, has not been reported until now. For such compositions, plastic relaxation takes place through a polygonal network of MDs, which have Burgers vectors in the interface plane. The origin of this network is an unusual dislocation source that occurs through the formation of a three-pointed star-shaped configuration. The characteristics of this misfit dislocation network, which has a higher misfit relieving component and a glide plane coincident with the interface plane, imply a reduction of the previous critical layer thickness estimates for high In content InGaAs/GaAs (111)B heterostructures. However, we observe that none of the (111)B samples shows evidence of a transition to a three-dimensional growth mode, which represents a significant advantage compared to the behavior of high In content quantum wells on (001) substrates. © 2002 American Institute of Physics.