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A DNA fragment d(GCGAAAGC), postulated to adopt a stable mini-hairpin structure on the basis of its extraordinary properties, has been X-ray analyzed. Two octamers related by a crystallographic twofold symmetry are aligned in an antiparallel fashion and associate to form a duplex, which is maintained by two Watson-Crick G.C base pairs and a subsequent sheared G.A pair at both ends. The central two A residues are free from base-pair formation. The corresponding base moieties of the two strands are intercalated and stacked on each other, forming a long column of G(1)-C(2)-G(3)-A(4)-A(5)(*)-A(5)-A(4)(*)-G(3)(*)-C(2)(*)-G(1)(*) (asterisks indicate the counter-strand). The Watson-Crick and major-groove sites of the four stacked adenine bases are exposed to the solvent region, suggesting a functional role. Since this structural motif is similar to those found in the nonamers d(G(Br)CGAAAGCT) and d(G(I)CGAAAGCT), the base-intercalated duplex may be a stable form of the specific sequence. Electrophoresis results suggest that the octamer has two states, monomeric and dimeric, in solution depending on the Mg(2+) concentration. The present duplex is preferred under the crystallization conditions, which correspond to physiologically allowed conditions.