The flavin-dependent enzyme FerB from P. denitrificans has been purified and both native and SeMet-substituted FerB have been crystallized. The two variants crystallized in two different crystallographic forms belonging to the monoclinic space group P2₁ and the orthorhombic space group P2₁2₁2, respectively. X-ray diffraction data were collected to 1.75 Å resolution for both forms.