Changes in the structure of PTFE produced by different cooling rates from the sintering temperature and following permanent mechanical deformation have been studied with the electron microscope. Slow cooling rates resulted in longer and wider striated bands (which constitute the PTFE) than fast cooling rates, however, there was no apparent change in the relative amount of crystalline material as x‐ray and infrared absorption studies indicate. Permanent strain resulted in severe distortion of the band structure; the striations either slipped past one another or became kinked. The manner in which the deformation occurred indicated it was controlled by a viscous noncrystalline phase. It is proposed that the band structure itself consists of two phases, crystalline striations or platelets separated by viscous noncrystalline matrix.