A series of ultrahigh modulus and strength polyethylene films of variable draw ratios (6–250), formed by the solid state coextrusion and post-drawing (SSE/PD of single crystal mats, were analysed by transmission electron microscopy (TEM). The high performance mechanical properties of these materials were attributed to the formation of ‘protofibrils’. Protofibrils are defined as long, highly crystalline fibrils which constitute a basic structural unit in ultradrawn polyethylene. The protofibrils observed here were made up of a series arrangement of ca. 8 nm thick ‘kebabs’ and crystalline ‘bridges’, were hundreds of nanometres long (along the chain direction), and were up to 10nm wide. Darkfield TEM, which yields a lower bound of crystal size, revealed crystalline regions with a protofibril substructure which were at least 3 microns long. The observed evolution of morphology with draw suggested that protofibrils formed via a stress-induced rearrangement of the original single crystal mat morphology and subsequent strain-induced crystallization.