The structure of the RNA octamer r(C4G4) has been determined in two different crystal forms. The conformations of the RNA duplex in the rhombohedral and hexagonal lattices display only minimal deviations, demonstrating that the RNA double helix is considerably less deformable than the DNA double helix. For the first time the crystal structures of an A-RNA and an A-DNA with identical sequence can now be compared. The large number of ordered water molecules observed in the minor groove of the RNA duplex suggests that an important contribution to its higher rigidity derives from the improved hydration due to the presence of the 2'-hydroxyl groups. Our finding that the conformation of the RNA double helix is virtually unaffected by different crystal packing modes provides evidence that proteins may not alter the conformation of RNA stem regions in a significant way.