The amino-terminal sequences of superoxide dismutase isolated from seven microorganisms have been determined. These include the first sequences of enzyme from anaerobic phototrophes. Five enzymes contain iron and two manganese. The enzymes are all related to each other but not to the Cu/Zn family of superoxide dismutases. These sequences, taken with six others from the same family, show that there is no clear distinction in sequence between Fe and Mn types. Moreover it demonstrates a wide variation between enzymes from different bacteria. Also enzymes from anaerobes do not seem to be a particularly closely related group and are not more closely related to each other than to enzymes from aerobes. Two histidine residues are conserved in all proteins and secondary structure predictions suggest they are in close proximity in the same alpha-helix. These residues may provide ligands for the bound metal.