Nucleic acid triplexes are formed by sequence-specific interactions between single-stranded polynucleotides and the double helix. These triplexes are implicated in genetic recombination in vivo and have application to areas that include genome analysis and antigene therapy. Despite the importance of the triple helix, only limited high-resolution structural information is available. The x-ray crystal structure of the oligonucleotide d(GGCGAATTGG) is described; it was designed to contain the d(G . GC)(2) fragment and thus provide the basic repeat unit oi a DNA triple helix. Parameters derived from this crystal structure have made it possible to construct models of both parallel and antiparallel triple helices.