Pseudomonas aeruginosa contains two superoxide dismutases (SOD), a Mn-containing SOD (Mn-SOD) and a Fe-SOD, which are encoded by sodA and sodB, respectively. We have cloned and sequenced a DNA fragment from P. aeruginosa, strain CHA, which contains the sodA gene and three other open reading frames (ORF). We report here that one of the ORFs upstream from sodA is fumC, which encodes the O2.- resistant isoform of fumarase (or fumarate hydratase). It is shown that fumC and sodA belong to the same operon. By primer extension experiments, the transcription initiation site has been located at -413 from the ATG codon of the fumC gene. The fumC-sodA operon was found to be negatively regulated in presence of iron and the E. coli FUR protein was shown to bind to the 19-bp FUR consensus sequence present at the transcription start site of the operon.