Abstract An electron microscope study has been made of mylonitized ultramafic rocks from Milford Sound, New Zealand. In the olivine porphyroclasts, substructural arrays of dislocations with Burgers vector b =  were observed. These substructures are similar to those observed in olivines deformed experimentally under creep conditions. Superimposed on the substructures was a variable density of free dislocations, b = , ranging from 10 7 cm −2 to 2 × 10 9 cm −2. The olivine grains in the matrix, while containing ordered arrays of dislocations, had relatively high densities of free dislocations, up to 5 × 10 9 cm −2. The orthopyroxene, as both porphyroclasts and matrix, contained fine-scale lamellae of clinoenstatite. The occurrence of the clinoenstatite suggested a low temperature fast strain rate phase of deformation. Although the metamorphic and tectonic histories of the region have been interpreted in terms of three phases of deformation under conditions of decreasing metamorphic grades, at least two of these phases have been verified by the crystallographic defects associated with these phases of deformation. It is proposed that the microstructural development of the ultramafic rocks occurred during the crustal deformation that overprinted the substructural evidence for the upper mantle origin of these rocks.