Using time-resolved X-ray diffraction at the European Synchrotron Radiation Facility we have observed a highly oriented weak transient diffraction peak which persists for about 0.2 s prior to strain-induced crystallization during the uniaxial drawing of poly(ethylene terephthalate) (PET) under industrial processing conditions. This structure may be identified with the mesophase structure proposed by a number of workers to occur during drawing of PET, poly(ethylene naphthalate) (PEN) and random copolymers of PET and PEN. In our studies, the transient structure was not observed at draw temperatures greater than 90 degrees C nor when the draw conditions resulted in a degree of polymer orientation below a critical level. The possibility that this transient structure is a precursor of strain-induced crystallization is suggested by our observation of a correlation between the decay of the diffraction associated with it and an increase in the intensity of diffraction peaks associated with the development of crystallistion.