synopsisTwo types of emulsion-grade polytetrafluoroethylene particles have been studied. We refer to these as ribbons and rods. The ribbons consist of very thin ribbons or lamellae folded upon themselves a number of times. In typical emulsion-grade material prepared at Allied Chemical, the unraveled ribbon measures about 3.25 p in length, 0.25 I( in width, and 60 d in thickness. The folded ribbons, which form the particles, are about 0.5 p long and 0.25 p wide. Electron diffraction shows that the ribbons are single crystals with the chain axis parallel to the long axis of the ribbons thus forming extended chain crystals. This extended-chain packing is consistent with the observed cleavage or fibrillation of the ribbons and with the molecular weight. The rods are formed in lowyield polymerizations. Electron diffraction also shows that the rods are single crystals with the chain axis parallel to the long axis of the rods. Striations parallel to the long axis are believed to result from stacking of parallel segments. Considerable bending of the long axis of rods is observed.