The three-dimensional structure of the iron-containing superoxide dismutase (EC 1.15.1.1) from Pseudomonas ovalis has been determined at 2.9-A resolution by the method of multiple isomorphous replacement. The molecule is a dimer of two identical subunits with the iron atom per monomer. The conformation of the enzyme is completely different from that of the eukaryotic copper-zinc superoxide dismutase. Each subunit consists of about 50% alpha-helix plus three strands of antiparallel pleated sheet. The iron atoms are coordinated by four protein ligands, one of which is the side-chain of histidine-26. Crystals of complexes with the inhibitors azide or fluoride are considerably more resistant to irradiation than those of the free enzyme. The structure of the apoprotein is identical to that of the iron-containing molecule.