Magnetotactic bacteria are a physiologically diverse group of prokaryotes whose main common features are the biomineralization of magnetosomes and magnetotaxis, the passive alignment and active motility along geomagnetic field lines. Magnetotactic bacteria exist in their highest numbers at or near the oxic–anoxic interfaces (OAI) of chemically stratified aquatic habitats that contain inverse concentration gradients of oxidants and reductants. Few species are in axenic culture and many have yet to be well described. The physiology of those that have been described appears to dictate their local ecology. Known Fe 3 O 4-producing strains are microaerophiles that fix atmospheric nitrogen, a process mediated by the oxygen-sensitive enzyme nitrogenase. Marine Fe3O4-producing strains oxidize reduced sulfur species to support autotrophy through the Calvin–Benson–Bassham or the reverse tricarboxylic acid cycle. These organisms must compete for reduced sulfur species with oxygen, which chemically oxidizes these compounds, and yet the organism still requires some oxygen to respire with to catalyze these geochemical reactions. Most Fe3O4-producing strains utilize nitrogen oxides as alternate electron acceptors, the reductions of which are catalyzed by oxygen-sensitive enzymes. Fe3O4-producing magnetotactic bacteria must solve several problems. They must find a location where both oxidant (oxygen) and reductants (e.g., reduced sulfur species) are available to the cell and therefore in close proximity. They must also mediate oxygen-sensitive, ancillary biochemical reactions (e.g., nitrogen fixation) important for survival. Thus, the OAI appears to be a perfect habitat for magnetotactic bacteria to thrive since microaerobic conditions are maintained and oxidant and reductant often overlap.