The structure and mechanical properties of cotton fibres UV-irradiated in the course of their maturation have been studied. In the wide angle meridional X-ray diffractograms a reflection is found at 2θ ≈ 19–20° when for the fibres grown in the usual natural conditions such a reflexion is not detected. A difference was also found in the strain and long-term properties of irradiated and non-irradiated fibres.