The ferredoxin (Fd I) (Mr2 X 6000, one (4 Fe-4 S) cluster per subunit) from the sulphate-reducing bacteria Desulfovibrio desulfuricans Norway 4 has been crystallized. The space group is P4(2)32 with a = 71.8 A. The two monomers of the molecule are probably related by a dyad axis.