Gel-filtration chromatography experiments performed at high protein concentrations demonstrate that the Cu,Zn superoxide dismutase from Escherichia coli is monomeric irrespective of the buffer and of ionic strength. The catalytic activity of the recombinant enzyme is comparable with that of eukaryotic isoenzymes, indicating that the dimeric structure commonly found in Cu,Zn superoxide dismutases is not necessary to ensure efficient catalysis. The analysis of the amino acid sequences suggests that an altered interaction between subunits occurs in all bacterial Cu,Zn superoxide dismutases. The substitution of hydrophobic residues with charged ones at positions located at the dimer interface of all known Cu,Zn superoxide dismutases could be specifically responsible for the monomeric structure of the E. coli enzyme.