In spite of the published claims to the contrary, Paracoccus denitrificans was shown to contain a heme derivative, virtually indistinguishable from the Escherichia coli heme O on the basis of the reverse-phase high-performance liquid chromatography and MALDI-TOF mass spectrometry analyses. Aeration of the anoxically grown culture resulted in the disappearance of a significant portion of this compound with concomitant build-up of heme A.