Very fine transformation twins with a thickness of 1-5 nm have been found in a b.c.t. martensite having an abnormally high tetragonality. The (112) boundaries of these ultrafine twins are fragmented and have many steps with a height of 1–6 interplanar distances of the (112) planes, which seems to be a result of the accommodation of local strains at the austenite-martensite interface on an atomic scale.