A very simple plastic deformation process was applied to thin films of ultra-high-molecular-weight polyethylene (UHMWPE) prepared from dilute solution and mounted on standard 100 mesh electron microscopy grids in order to study the changes in supermolecular morphology which occurred during necking. Starting from an as-cast spherulitic type of microstructure, processes such as macromolecular reorientation, the destruction of lamellar crystallinity and the initial stage of microfibril formation were directly observed in the transmission electron microscope. Microfibrils were found to be composed of 10 nm thick crystals, in contrast to an initial lamellar thickness of 20 nm for the original microstructure. It is believed that the new crystalline morphology was generated by a process of complete lamellar destruction followed by the stress-induced recrystallization of extended chain segments.