TRIPLE helices result from interaction between single- and double-stranded nucleic acids. Their formation is a possible mechanism for recombination of homologous gene sequences in nature and provides, inter alia, a basis for artificial control of gene activity, Triple-helix motifs have been extensively studied by a variety of techniques, but few high-resolution structural data are available, The only triplet structures characterized so far by X-ray diffraction were in protein-DNA complexes(1,2) studied at about 3 Angstrom resolution, We report here the X-ray analysis of a DNA nonamer, d(GCGAATTCG), to a resolution of 2.05 Angstrom, in which the extended crystal structure contains (C . G)*G triplets as a fragment of triple helix. The guanosine-containing chains are in a parallel orientation, This arrangement is a necessary feature of models for homologous recombination which results ultimately in replacement of one length of DNA by another of similar sequence, The present-structure agrees with many published predictions of tripler organization, and provides an accurate representation of an element that allows sequence-specific association between single- and double-stranded nucleic acids.