Membrane fragments prepared from Micrococcus denitrificans grown aerobically in an iron-sufficient medium showed an ESR signal at g=1.94 on reduction with NADH, but not with succinate. From the properties of this signal, it was concluded that the NADH dehydrogenase region of the respiratory chain of such preparations contained a non-heme iron similar to that occurring in mammalian mitochondria. The signal at g=1.94 was greatly reduced or hardly detectable in membrane fragments prepared from cells grown in an iron-deficient medium (o-phenanthroline.