Apparatus is described which enables strains of up to 1000% to be applied to a specimen during observation in the electron microscope. The device, which was designed for use with rubbers and other highly-extensible materials, is made from an ordinary specimen capsule and requires no modification of the microscope. It is operated by means of the stereo push rod and employs a lever system to separate a pair of sliding jaws. An example of its use is the observation of strain-induced crystallinity in natural rubber.