The solution structure of the dimeric N-terminal domain of HIV-2 integrase (residues 1-55, named IN(1-55)) has been determined using NMR spectroscopy. The structure of the monomer, which was already reported previously [Eijkelenboom et al. (1997) Curr. Biol., 7, 739-746], consists of four alpha-helices and is well defined. Helices alpha1, alpha2 and alpha3 form a three-helix bundle that is stabilized by zinc binding to His12, His16, Cys40 and Cys43. The dimer interface is formed by the N-terminal tail and the first half of helix alpha3. The orientation of the two monomeric units with respect to each other shows considerable variation. 15N relaxation studies have been used to characterize the nature of the intermonomeric disorder. Comparison of the dimer interface with that of the well-defined dimer interface of HIV-1 IN(1-55) shows that the latter is stabilized by additional hydrophobic interactions and a potential salt bridge. Similar interactions cannot be formed in HIV-2 IN(1-55) [Cai et al. (1997) Nat. Struct. Biol., 4, 567-577], where the corresponding residues are positively charged and neutral ones.