In aqueous solution, pure DNA forms multiple liquid crystalline and crystalline phases whose nature depends on the polymer concentration. The following phase sequence is observed when the DNA concentration increases : isotropic ↦ cholesteric ↦ columnar hexagonal ↦ crystalline phases. The aim of this work is to obtain structural information about the highly concentrated phases formed by 500 Å long DNA molecules — in particular about the crystalline phases — by means of X-ray diffraction. We show that in the two-dimensional (2D) ordered hexagonal phase a longitudinal order progressively appears between neighbouring DNA helices leading continuously to a three-dimensional (3D) ordered hexagonal phase. For higher concentrations the specimens undergo a discontinuous transition towards an orthorhombic phase. The characteristic structural parameters of these different phases have been determined. An important result is that the number of nucleotides per helix turn decreases continuously, when the DNA concentration increases, from 10.3±0.1 at the cholesteric ↦ hexagonal transition down to 9±0.1 without any apparent change of the B conformation of the molecules.