In nitrite-treated cytochrome cd(1) nitrite reductase, heme d(1) is electron paramagnetic resonance silent but paramagnetic. Analysis of the unusual temperature dependence of the magnetic circular dichroism spectra unambiguously demonstrates that the heme d(1) is not in the oxoferryl (Fe-IV=O) state but is low-spin Fe-III weakly coupled to a radical species. This species could be either a protein-bound radical generated by a nitrite ion reacting with a heme group resulting in a one-electron oxidation of an amino acid residue, possibly tyrosine or tryptophan, adjacent to heme d(1) or a heme d(1) (FeNO)-N-III complex.