It is a well-known experimental observation that the melting transition of polymer crystals can be affected by an external constraint, force or pressure. This effect is enhanced in cases in which chain extension is maintained. In particular, the melting temperature of extended-chain polyethylene fibers is elevated by increased hydrostatic pressure. We use the theoretical description of Elyashevich, Baranov, and Frenkel to analyze the effect of chain extension on the relative increase in the melting temperature. This model considers explicitly the entropy difference between the crystal and melt when a given degree of chain extension is maintained. We extend this interpretation to consider the melting of different metastable phases, using the melting enthalpy as a parameter. This can explain the observed metastability of the hexagonal phase of polyethylene in extended-chain fibers under moderate pressure, as recently observed. (C) 2004 Wiley Periodicals, Inc.