When DNA fibres are stretched during drying, the polymer undergoes a conformational transition. We present quantitative results from X-ray diffraction studies on such fibres held at various ambient relative humidities. These indicate that the molecules are arranged in arrays which are crystalline in projection down the fibre axis. The packing can be explained in terms of a hexagonal cell with a lattice parameter, a , of ∼13 A which varies with humidity. The patterns contain meridional intensities at 1/3.4 A −1 and 1/6.5 A −1 , a strong off-meridional intensity at Z =1/5.6 A −1 and diffuse scatter at Z =1/28 A −1 .
