The first antiviral nucleoside 5-iodo-2'-deoxyuridine (IDU) against herpes simplex virus type 1 and type 2 is a thymidine analogue, i.e. the C5 methyl group is replaced by an I atom. The structure of the self-complementary hexamer d[CACG(IDU)G] was determined by single-crystal X-ray diffraction techniques. The orthorhombic crystals belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 18.16, b = 30.03, c = 41.99 Angstrom. Refinement in the resolution range 20 - 1.3 Angstrom converged with a final R1 = 0.167, including 43 water molecules and two cobalt hexammine complexes. The incorporation of a large I atom has only minor consequences for the overall structure as is noticed in the IDU . A base pairs, which are of the common Watson - Crick type. To contribute to the still puzzling mechanism of this historically important agent, details of base stacking, helical parameters, hydration etc. have been studied. A general scheme of cobalt hexammine-binding modes in Z-DNA is provided, revealing similar binding modes for the reported structure.