Crystallization of a DNA double helix with overhanging bases at the 5'-ends of both strands, results in the formation of two crystallographically independent (C.G)*G triplets. In a previous report [Van Meervelt, Vlieghe, Dautant, Gallois, Précigoux & Kennard (1995). Nature (London), 374, 742-744] the unique molecular packing of the duplex and the Hoogsteen hydrogen-bond pattern and parallel backbone orientation of the guanine-containing strands in the triplets was described. The fine structural details and hydration of the d(GCGAATTCG) crystal structure refined to 2.05 A (R = 0.168, 86 water molecules, two Mg(2+) cations) are now presented. Helical parameters, stacking effects, the geometry at the duplex-triplex junction, and the hydration of the minor groove are discussed and compared with related theoretical and crystal structures.