The ArcA and Fnr regulators of Escherichia coli, both of which are activated in anaerobic conditions, negatively regulate the sodA gene (coding for manganese superoxide dismutase), but Fnr has no effect on anaerobic sodA expression in a delta arcA delta fnr background (Compan and Touati, 1993). We show here that the sdh gene (coding for succinate dehydrogenase) is also negatively regulated by Fnr, but again Fnr exerts no control in a delta arcA background. One interpretation of these results is that Fnr activates arcA transcription. Using arcA-lac transcriptional and translational fusions, we show that arcA expression increases (about fourfold) in anaerobiosis and that both Fnr and ArcA are required for full expression. In a delta fnr background, there is no autoactivation, suggesting that ArcA enhances activation by Fnr. Transcript and sequence analyses reveal that the arcA upstream regulatory region lies within a 530 bp non-coding DNA fragment, which contains five putative promoter sequences and a putative Fnr-binding site. Identification of the transcription start sites indicates that transcription occurs in aerobiosis from three constitutive upstream promoters (Pe, Pd, Pc). In anaerobiosis an additional completely Fnr-dependent transcript starting at Pa is present; expression from Pa is reduced in the absence of ArcA, and Fnr activation at Pa blocks the weak anaerobic-dependent expression from Pb. Fnr activation of arcA transcription may play an important role in the co-ordination of expression of genes associated with aerobic and anaerobic metabolism during environmental changes.