A novel application of in situ neutron diffraction under applied uniaxial strain is presented; measuring the crystalline domain evolution in a semi-crystalline polymer under bulk deformation. PTFE is shown to respond to uniaxial deformation by undergoing a crystalline phase transition that is previously believed to occur only at very high hydrostatic pressure. Discovery of this phase transition under applied uniaxial strain fundamentally changes our understanding of the deformation mechanisms in semi-crystalline polymers and how they need to be modeled. Under compression parallel to the basal plane normal (i.e., parallel to the molecular axis) the modulus is similar to 1000 x bulk dominated by intra-polymer chain compression, providing experimental validation of theoretical predictions. Deformation parallel to the pyramidal plane normal exhibits both axial and transverse strains of the opposite sign as the applied load, suggesting that the crystalline lattice is accommodating deformation by shearing along the prismatic planes. Published by Elsevier Ltd.