TNA (alpha-(L)-threofuranosyl-(3'-2') nucleic acid) is a nucleic acid in which the ribofuranose building block of the natural nucleic acid RNA is replaced by the tetrofuranose alpha-(L)-threose. This shortens the repetitive unit of the backbone by one bond as compared to the natural systems. Among the alternative nucleic acid structures studied so far in our laboratories in the etiological context, TNA is the only one that exhibits Watson-Crick pairing not only with itself but also with DNA and, even more strongly, with RNA. Using NMR spectroscopy, we have determined the structure of a duplex consisting entirely of TNA nucleotides. The TNA octamer (3'-2')-CGAATTCG forms a right-handed double helix with antiparallel strands paired according to the Watson-Crick mode. The dominant conformation of the sugar units has the 2'- and 3'-phosphodiester substituents in quasi-diaxial position and corresponds to a 4'-exo puckering. With 5.85 angstrom, the average sequential P-i-Pi+1 distances of TNA are shorter than for A-type DNA (6.2 angstrom). The helix parameters, in particular the slide and x-displacement, as well as the shallow and wide minor groove, place the TNA duplex in the structural vicinity of A-type DNA and RNA.