The question of the dominant factors in the production of a transcrystalline region (TCR), an ＇＇interphase,＇＇ in crystalline polymers on solidifying in contact with a substrate is investigated for isotactic polypropylene (PP) using polarization microscopy and scanning electron microscopy. Several uncoated substrates, and the same substrates coated with certain vacuum-evaporated metals and carbon, as well as surface replicas of these made with cellulose acetate, are compared as nucleating surfaces. It is concluded that TCR formation is not dependent on the species of substrates or their surface energy but on the geometrical morphology or surface roughness. In fact, the rough surface produced by abrasion with carborundum powder creates a TCR for every substrate used. An all-transcrystalline (TC) film of PP more than 300 mu m thick is obtained by hot pressing the PP between two sheets of PTFE. Wide-and small-angle X-ray analysis of the film reveals a ＇＇cross-hatch＇＇ structure of lamellae. Micro-beam X-ray analysis shows that three-dimensional crystal growth occurs up to 20-30 mu m from the interface and, thereafter, a one-dimensional TCR grows up to the mid-plane of the film from both sides. The stress-strain behavior of the various TC films are compared with spherilitic-crystalline and quenched amorphous films. The TC film is able to undergo up to 800% elongation in spite of its almost perfect crystallinity. The mechanism of deformation is discussed on the basis of X-ray observations.