The regulatory protein Fnr is required for anaerobic expression of several anaerobic respiratory enzymes in Escherichia coli. To gain insight into how Fnr activity is regulated by oxygen, we have isolated Fnr mutants that increase expression of the nitrate reductase operon in the presence of oxygen (Fnr* mutants). Seven single-amino-acid substitutions that mapped within two regions of Fnr have been characterized. Two mutants mapped adjacent to two Cys residues in the N-terminal Cys cluster. Five Fnr* substitutions mapped to a region of Fnr that is similar to the cyclic AMP-binding domain of the catabolite activator protein (CAP). Within this group, four mutants were clustered in a region analogous to the CAP C helix, which is important in CAP dimer subunit interactions. Taken together, these data implicate regions in Fnr that may be important either in sensing oxygen deprivation or in the conformational change proposed to be necessary for Fnr activation under anaerobic conditions.