Abstract Prismatic sillimanite (Al 2SiO 5), with a length between 0.3 and 2.5 mm, was obtained from a garnet migmatite. The sillimanite, naturally deformed at a temperature of 750 ° C and a confining pressure of 6 kbar, has been studied using optical and transmission electron microscopy techniques. Optical and universal stage measurements reveal undulatory extinction, “sharp” deformation-induced subgrain boundaries (subparallel to (001) and (010)) and minor recrystallization. Transmission electron microscopy shows free dislocations, dislocation loops, (110) planar defects and tiltwalls parallel to (001). Dislocations have Burgers vectors of  and . All isolated 〈c〉 dislocations are dissociated. The dominant slip system is (100)  with subordinate (001) . The microstructure of sillimanite indicates that recrystallization has occurred by a rotation mechanism (around ), where single crystals become polycrystals by the progressive development of numerous internal high-angle boundaries. The latter have been interpreted as originating from low-angle (001) tilt- and (010) twistwalls. Rotation recrystallization was followed by grain boundary migration.