99.15 pure zinc, when rolled at room temperature to reductions in excess of 50cp, shows properties grouped under the general title of 'extended plasticityl . At these reductions, the flow stress and elongation show an increasing strain-rate sensitivity, and at 8 reduction, at which most of the tests reported here were done, the conventional lar7e twinned. Grains were converted to a stable sub-grain structure 2-3 microns in diameter. This structure remains apparently unchanged after about 120;' elongation in uniaxial tension; no Evidence of slip or twinning has so far been observed under the electron microscope. Work to elucidate the deformation mechanism is continuing.