The chromonic lyotropic liquid crystal formed from an aqueous solution of the azo dye C.I. Direct Blue 67 was investigated by means of a temperature-controlled X-ray diffraction technique, polarized optical microscopy, and UV-visible spectroscopy. The. dye molecules form columnar aggregates even at low concentrations. The stacking distance of 3.4 Angstrom was observed independent of the concentration and temperature. The addition of a small amount of anionic surfactant was found to induce a nematic phase from a more-ordered hexagonal phase and, thus, enhanced the stability of the nematic phase within a wide range of dye concentrations. The cross-sectional diameter of a column was found to be in good agreement with the molecular length of the dye, indicating that the column is comprised of unimolecular stacking.