The crystal structure of Escherichia coli MobB, an enzyme involved in the final step of molybdenum-cofactor biosynthesis, forms intertwined dimers. Each molecule consists of two segments and requires the second monomer for stable folding. Dimerization buries a quarter of the solvent-accessible area of the monomer. These dimers assemble into a hexagonal lattice with P6(4)22 symmetry and occupy only approximately 25% of the unit-cell volume. The symmetry-related dimers associate tightly into a helical structure with a diameter of 250 A and a pitch of 98 A. Two such helices are intertwined, shifted by 49 A along the sixfold axis. Within the crystal, these helices form thin-walled cylinders with an external diameter of 250 A and an internal diameter of 190 A. Their center is filled with solvent. These cylinders pack closely together, forming a hexagonal lattice with the highest possible packing density. This arrangement of dimers allows extensive intermolecular contacts with 75% solvent content in the crystal.