Sulfate-reducing bacteria (SRB), which are strict anaerobes, contain an electron-transfer chain from pyridine nucleotides to molecular oxygen. This unique enzymatic equipment allows the bacterium to produce ATP when exposed to air from the degradation of internal reserves of polyglucose. Ferredoxin II (Fd II) is a small electron-transfer protein isolated from the strict anaerobic sulfate-reducing bacterium Desulfovibrio gigas. The protein contains 58 amino acids and an iron-sulfur cluster. The cluster [3Fe-4S] spontaneously undergoes conversion to [4Fe-4S] when it is used as an electron mediator in the phosphoroclastic reaction. The iron-sulfur geometries and interconversion mechanism appear to have physiological significance between the oxidized and reduced states. Crystallization of Fd II in an anaerobic environment was achieved at a higher resolution of 1.37 A and the differences between the anaerobic and aerobic structures will reveal the unique iron-storage function and electron-transfer mechanism of ferredoxin II from D. gigas.