The occurrence and distribution of magnetotactic bacteria (MB) were studied as a function of the physical and chemical conditions in meromictic Salt Pond, Falmouth, Mass., throughout summer 2002. Three dominant MB morphotypes were observed to occur within the chemocline. Small microaerophilic magnetite-producing cocci were present at the top of the chemocline, while a greigite-producing packet-forming bacterium occurred at the base of the chemocline. The distributions of these groups displayed sharp changes in abundance over small length scales within the water column as well as strong seasonal fluctuations in population abundance. We identified a novel, greigite-producing rod in the sulfidic hypolimnion that was present in relatively constant abundance over the course of the season. This rod is the first MB that appears to belong to the ␥-Proteobacteria, which may suggest an iron-rather than sulfur-based respiratory metabolism. Its distribution and phylogenetic identity suggest that an alternative model for the ecological and physiological role of magnetotaxis is needed for greigite-producing MB.Magnetotactic bacteria (MB) are motile gram-negative bacteria whose directional swimming behavior is affected by the Earth's geomagnetic and external magnetic fields. They contain highly ordered intracellular chains of magnetic iron minerals, either magnetite (Fe 3 O 4 ), greigite (Fe 3 S 4 ) (33), or in one case, both (7). Magnetite MB (MMB) are found in both freshwater and marine environments, while greigite MB (GMB) appear to be unique to marine systems. Both MMB and GMB are present in salt marsh sediments, stratified salt ponds, estuarine basins in the Eastern coastal United States (5-7, 32, 33, 51), coastal marshes along the California coast (E. F. DeLong, personal communication), and other similar habitats worldwide, as well as deep-sea sediments (43). This widespread distribution of marine MB suggests that they are globally more abundant than freshwater MB.All known MB share the following characteristics: motility by means of flagella, negative tactic responses to atmospheric concentrations of oxygen, optimal growth under microaerophilic or anaerobic conditions, and a respiratory metabolism (51). MB are typically found at or below the oxycline in sediments and water columns and can reach significant population densities: the freshwater MMB "Magnetotacticum bavaricum" (proposed name) can reach 7 ϫ 10 5 cells per cm 3 at the oxycline in freshwater sediments (48). Many, if not most, groups of MB that have been observed in the environment have not been grown in pure culture in the laboratory. Fewer than 10 strains are available in pure culture, and most of those are magnetiteproducing ␣-Proteobacteria of the genus Magnetospirillum from freshwater environments. A few strains of marine MMB are available in culture (3,45,51). These strains are all microaerophiles when growing on O 2 , and some can grow anaerobically on N 2 O (3). All are capable of oxidizing but not reducing inorganic sulfur compounds, with the exceptio...