A range of polyethylenes has been studied by X-ray diffraction during deformation. Changes in the crystalline orientation have been related to the load-extension curve. This was altered by changing the molecular weight, concentration of branches and strain rate. Crystalline orientation was first observed just after the maximum load, before any visible signs of yield. At about the same point, the monoclinic phase was observed in specimens that were subjected to a higher load and it disappeared when stress softening occurred. Initially the (110) orthorhombic reflection oriented to give four maxima, which implies tilted polymer chains and lamellae. Subsequently the (110) concentrated on the equator showing chains oriented parallel to the draw direction. The transition from tilted to fibre orientation occurred at different points on a nominal stress-strain curve but at approximately the same local strain, as estimated from the change in sample dimensions.