Dynamic plane-stress failure has been observed directly in the transmission electron microscope as a function of temperature using a commercially available cooling/straining holder in conjunction with a copper deformation cartridge. The low-temperature cooling stage permits studies of the ductile-brittle transition when the transition is between +23 and -170 degrees C. A change in deformation mode was observed on the submicrometre level for polypropylene and impact-modified polypropylene. At room temperature, polypropylene and impact-modified polypropylene deform by shear yielding. Below the ductile-brittle transition the polymer chain mobility is curtailed and crazing dominates. The stage allows observation of the deformation at varying temperature, and events occurring during the deformation can be recorded in real time using a CCD camera.