An experimental set-up in the synchrotron radiation beam was constructed for performing wide angle X-ray scattering (WAXS) on-line measurements, during high speed spinning of poly(ethylene terephtalate) (PET). In order to detect the very weak scattering of the thin filaments (= 26 mu m diameter) the capton windows of the vacuum tubes were removed and the air was replaced by helium. In this way it became possible to measure the development of crystal reflections as a function of the distance from the spinneret. A comparison of the results with corresponding on-line measurements of the fibre diameter indicates that crystallization starts at the beginning of the necking zone.