In the Twin Sisters ultramafic massif, NW Washington, an similar to 0 center dot 5 cm thick, isolated ultramafic ultramylonitic shear zone displaces orthopyroxenite and clinopyroxenite dikes, by a minimum of 21 cm. The shear zone exists only adjacent to the orthopyroxenite and clinopyroxenite dikes, with deformation distributed along strike into the wall-rock less than 10 cm from the dikes in the outcrop face. Microstructurally, the shear zone contains domains of different grain sizes and phase proportions. A marginal domain of almost pure olivine displays a mean grain size of similar to 30-100 mu m and an olivine lattice preferred orientation (LPO) indicating that glide occurred on (010)[100] and (010)[001]. A central domain of mixed olivine, pyroxene and amphibole displays a finer grain size, ranging down to similar to 5 mu m. Aligned grain and phase boundaries and weak olivine and pyroxene LPOs indicate that this zone deformed by a grain-size-sensitive deformation mechanism (e.g. grain boundary sliding accompanied by diffusion creep). Geothermometry indicates that shearing occurred at temperatures in the range 650-750 degrees C. We interpret the formation of this ultramylonitic shear zone as a shear instability caused by the presence of compositional heterogeneity. Localization was promoted by a deformation mechanism switch from dislocation creep to grain-size-sensitive creep as a result of syn-deformational grain-size reduction. Mineral compositions indicate that this grain-size reduction was associated with reaction. The fine grain size was stabilized by the presence of multiple phases, particularly pyroxene, within the central shear zone domain. The shear zone did not propagate beyond the vicinity of the pyroxenite dikes because the fine grain sizes necessary for the deformation mechanism switch to occur could not be maintained in the monophase olivine forming the surrounding dunite.