A DNA fragment d(GCGAAAGCT), known to adopt a stable mini-hairpin structure in solution, has been crystallized under a slightly acidic condition, and its crystal structure has been determined at 2.5A resolution. In the first half of the oligomer, the sequence CGAA forms a parallel duplex with another symmetry-related half through the homo base pairs, C2:C2+ (semi-protonated between the Watson-Crick sites), G3:G3 (between the minor groove sites), A4:A4 (between the major groove sites) and A5:A5 (between the Watson-Crick sites). The second halves of the parallel duplex are split away in the different direction, and extended to form an anti-parallel B-form duplex with another second half.