We report the NMR structure of the DNA sequence d-TGGGCGGT in Na+ solutions at neutral pH, containing a repeat sequence from SV40 viral genome. The structure is a novel quadruplex incorporating the C-tetrad formed by symmetrical pairing of four Cs via NH2-O-2 H-bonds in a plane, The C-tetrad has a wider cavity compared to G-tetrads and stacks well over the adjacent G4-tetrad, but poorly on the G6 tetrad, The quadruplex helix is largely underwound by 8-10 degrees compared to B-DNA except at the C5-G6 step. To our knowledge this is the first report of C-tetrad formation in DNA structures, and would be of significance from the point of view of both structural diversity and specific recognition.