A bacterioferritin was recently isolated from the anaerobic sulphate-reducing bacterium Desulfivibrio desulfuricans ATCC 27774 [Romão et al. (2000) Biochemistry 39, 6841-6849]. Although its properties are in general similar to those of the other bacterioferritins, it contains a haem quite distinct from the haem B, found in bacterioferritins from aerobic organisms. Using visible and NMR spectroscopies, as well as mass spectrometry analysis, the haem is now unambiguously identified as iron-coproporphyrin III, the first example of such a prosthetic group in a biological system. This unexpected finding is discussed in the framework of haem biosynthetic pathways in anaerobes and particularly in sulphate-reducing bacteria.