A new crystal modification induced by strain was found in uniaxially oriented fibres of poly(tetramethylene succinate). This new modification (beta form) had a planar zigzag conformation of T10 (all trans) and a fibre identity period of 11.90 angstrom, while the conventional alpha form had a T7GTGBAR conformation and fibre identity period of 10.90 angstrom. The solid-state crystal transition between alpha and beta forms was reversible with respect to the successive application and release of strain.