Molecular order and motion in isotactic and atactic polypropylene molecules have been studied by proton magnetic resonance methods over the temperature range 77–400°K, and by x‐ray diffraction. It is shown that some motion persists at the lowest temperatures, and becomes pronounced in the temperature interval 77–110°K. This behavior is ascribed to motion of the methyl groups about the threefold axis. The resonance undergoes further narrowing near room temperature for the atactic polymer, presumably owing to rotational and translational motions of chain segments. The isotactic compound possesses a narrow resonance superposed on a broad absorption over a wide temperature range. The former, ascribed to the amorphous regions, narrows at higher temperatures than for the atactic polymer, presumably owing to constraints imposed by the crystalline regions. The chains of the isotactic compound are less mobile than in polyethylene, as shown by the resonance studies and by x‐ray diffraction comparisons of the rate of crystallization.