Shewanella oneidensis is a dissimilatory iron reducing bacterium capable of inducing the extracellular precipitation of magnetite. This precipitation requires a combination of passive and active mechanisms. Precipitation occurs as a consequence of active production of Fe 2+ (aq) when bacteria utilize ferrihydrite as a terminal electron acceptor, and the pH rise probably due to the bacterial metabolism of amino acids. As for passive mechanisms, the localized concentration of Fe 2+ (aq) and Fe 3+ (aq) at the net negatively charged cell wall, cell structures and/or cell debris induces a local rise of supersaturation of the system with respect to magnetite, triggering the precipitation of such a phase.These biologically induced magnetites are morphologically identical to those formed inorganically in free-drift experiments (closed system; 25°C, 1atm total pressure), both from aqueous solutions containing Fe(ClO 4 ) 2 , FeCl 3 , NaHCO 3 , NaCO 3 and NaOH, and also from sterile culture medium added with FeCl 2 . However, organic material becomes incorporated in substantial amounts into the crystal structure of S. oneidensis-induced magnetites, modifying such a structure compared to that of inorganic magnetites. This structural change and the presence of organic matter are detected by Raman and FT-IR spectroscopic analyses and may be used as a biomarker to recognize the biogenic origin of natural magnetites.