The effect on PTFE of crystallization conditions and subsequent tensile deformation have been examined by Fourier analysis of the shapes of diffraction peaks. Crystal perfection increases with decreasing rate of cooling from the melt. During deformation, the crystallite size is reduced drastically in the range of strain from 35 to 185%. The microstrains are not altered over this range, implying crystal breakup rather than distortion within crystallites. It is in this range that previous studies with the same specimens have indicate a sharp transition in texture. The fact that root‐mean‐square (rms) microstrains increase due to elongation suggests that the crystalline regions not only break up but that they are also deforming plastically. Electron microscopic investigation indicate that the striation spacing on crystals in the fracture faces is much smaller than the crystallite size measured with x‐rays; thus the striations do not represent alternating crystalline and amorphous regions.