In 1910, Bang' reported that concentrated solutions of guanylic acid formed a gel. We have also observed that concentrated solutions (25.0 mg/ml) of guanylic acid (GMP) at pH 5 are extremely viscous and, if cooled, form a clear gel. Less concentrated solutions also gel on cooling but assume a more normal viscosity at room temperature. From examination of the optical properties of the gel and investigation of the structure of fibers obtained from the gel by drying, we have concluded that, at least in the case of the 5' isomer, the phenomenon may be explained as being due to helix formation by the guanylic acid. A possible structure is presented for this helix.