A bipolarly flagellated magnetotactic spirillum containing intracellular chains of single domain-sized magnetite crystals was isolated by applying a magnetic field to sediments from a freshwater swamp. The organism was cultured in a chemically defined medium containing ferric quinate and succinate as sources of iron and carbon, respectively. Nonmagnetic variants of this isolate were maintained in chemically defined medium lacking ferric quinate. In contrast to magnetic cells, these had less iron and lacked measurable magnetic remanence and the intracytoplasmic crystals. In other respects, including moles percent guanine plus cytosine content, growth characteristics, nutrition, and physiology, the two types were similar. The isolate reduced nitrate without accumulating nitrite and produced ammonia during growth. Nitrate or ammonium ions served as a nitrogen source. The organism was microaerophilic and did not grow anaerobically with nitrate in the medium. In chemically defined medium, cells synthesized magnetite only if the initial O2 concentration in the atmosphere of sealed cultures was 6% (vol/vol) or less.