The melting transition of A- and B-DNA has been investigated by wide-angle X-ray diffraction. A significant crystalline phase is present in both the systems, even if the fibers have not been artificially aligned. The behavior of the intramolecular Bragg peaks of both A- and B-DNA as a function of the temperature clearly reveals the unfolding structural transition of the double helix. This transition occurs at the same temperature as the melting of the crystalline phase. The trends of the intramolecular correlations and the index of crystallinity are nicely described by the Peyrard-Bishop-Dauxois model for DNA melting. A description of the processes taking place at a microscopic level, i.e., double-helix deformation, crystalline dilation, and collapse, on approaching and during thermal melting is proposed.