Crystals of the molbindin ModG (subunit Mr = 14359 Da), a cytoplasmic molybdate-binding protein from Azotobacter vinelandii, were grown by vapour diffusion. Both apo and tungstate-bound forms were crystallized and X-ray data were collected at 100 K. Apo-ModG crystallizes in space group P6322, with unit-cell dimensions a = b = 90.62, c = 79.46 A. Native data to a resolution of 2.5 A were collected from a single crystal, which showed a marked improvement in diffraction quality after annealing. Data from a single-site gold derivative were also collected at 2.7 A resolution. Crystals of the ligand-bound form of ModG belong to space group P321, with unit-cell parameters a = b = 50.57, c = 79.29 A. X-ray data to a resolution of 2.0 A were collected.