The objective of this study was to synthesize magnetite (Fe₃O₄) nanoparticles using diverse Shewanella species isolated from different environments. Magnetite formation experiments were performed with 11 species of Shewanella using akaganeite (β-FeOOH) as an electron acceptor and lactate (C₃H6O₃) as an electron donor under a N₂ atmosphere at room temperature. Magnetites and other products formed by the bacteria were characterized by XRD and TEM-EDS analyses. In this study, all the strains of Shewanella species produced magnetite nanoparticles with 2.5 to 20 nm in size. However, the size of the magnetite varied with the species of Shewanella, and a few species formed Fe(III) oxide as secondary minerals such as goethite and lepidocrocite. These results indicate that different species of iron-reducing bacteria belonging to the genus Shewanella exhibit different rates of Fe(III) reduction resulting in magnetite nanocrystals of varying size and formation of secondary mineral species.