Kittel <1946> gave the original presentation of the theory of the single-domain (SD) structure of fine ferromagnetic particles and their response to an applied magnetic field. The theory predicted that the SD coercive fields would be determined by the crystalline and shape anisotropies of the particle and could be much higher than those for the multidomain specimens used in magnetic recording at that time. The current magnetic recording industry has developed the use of magnetic fine particles. In geophysics, the observational evidence for plate tectonics, continental drift, and reversals rely, in lava flows, on the thermoremanent magnetization acquired on cooling through the Curie temperature. In sediments where physical deposition of magnetic particles occur, the measurements are of the magnetic orientations at times of deposition or soon thereafter of single-domain grains of inorganic or biogenic origin. To obtain the maximum total magnetic moment in a magnetobacterium, it is counterproductive for magnetosome growth to exceed the single-domain size limit. If in future extraterrestrial single-domain magnetic particles are found that are characteristic of magnetotactic bacteria, as in meteorite arrivals from Mars or other planets, they could be cited as evidence for extraterrestrial life. ¿ 1998 American Geophysical Union