Abstract The 3-Fe ferredoxin (FdII) from the bacterium Desulfovibrio gigas has been crystallized at pH 5.0 and 23 °C in two different crystal forms. One form is monoclinic, space group C2, with unit cell parameters a = 40.78 A ̊ , b = 44.98 A ̊ , c, = 26.47 A ̊ , β = 104.6 °C , and one monomer of the FdII tetramer per asymmetric unit. The molecule can be either the monomer of molecular weight 6400 or a dimer of twice this molecular weight with 2-fold symmetry coincident with the 2-fold axis of the crystal. The other crystal form is orthorhombic, space group P2 12 12 and unit cell parameters a = 109.5 A ̊ , b = 37.0 A ̊ , c = 30.5 A ̊ . The asymmetric unit of this crystal contains two monomers of FdII. The orthorhombic crystal has not been reproduced since the initial crystallization.