Abstract X-ray fibre diffraction studies have shown that at pH 7.0 polyribocytidylic acid (poly(C)) can adopt a single-stranded, right-handed, 6-fold helical structure with an axial rise per residue of 0.311 nm. The furanose rings have the C-3′-endo conformation. The other conformation angles of this structure are also remarkably similar to those observed in A -type double-stranded polynucleotide structures with the exception of θ[C-4′,z.sbnd;C-3′,z.sbnd;O-1,z.sbnd;P], which adopts a value very close to the monomer average. There is no hydrogen bonding between bases. The molecular structure appears to be stabilised primarily by base-stacking interactions and the crystal structure by hydrogen bonds between ribose hydroxyl groups.