The transformation from the orthorhombic to the monoclinic structure has been observed by means of infrared spectroscopy in shaken preparations of single crystals of polyethylene, mixed crystals of polyethylene and perdeuteropolyethylene, and physical mixtures of single crystals of these two polymers. Annealing of such shaken samples results in the conversion of the monoclinic back to the orthorhombic structure, but with an accompanying change from the (110) folding characteristic of single crystals to (200) folding. The monoclinic structure thus appears to be an intermediate state in the fold-plane transformation which can occur upon deformation. Both unannealed and annealed physical mixtures give evidence of ciliary penetration.