Optical and x‐ray observations of polyethylene have been made at high pressures and temperatures using a gasketed diamond‐anvil cell. The experiments confirm the existence of the high‐pressure phase previously postulated by Bassett and Turner. The new phase is hexagonal, with orthohexagonal lattice parameters ofa= 8.46Å andb= 4.88Å. Comparison with the previously measured volume change indicates that there is a decrease in thecdimension to 2.45Å per ethylene unit in transforming from orthorhombic to hexagonal structures. The likely implication is that the molecules in the hexagonal phase do not have an all‐trans conformation. Chain‐extended growth is the result of crystallization from the melt into the hexagonal phase, whereas chain‐folded growth is the familiar process of melt crystallization.into the orthorhombic phase. Chain‐extended lamellae are observed to grow outwards behind a growing edge with a permanent narrowed profile, showing that the lamellar thickness is determined in a region extending several microns behind the growth front.