The magnetic properties of certain soils indicate the widespread presence of ultrafine-grained magnetite, even where there is no detrital input of magnetite1-3. This suggests that in situformation of magnetite can occur under soil-forming conditions. Schwertmann and Taylor4 have stated, however, that magnetite has not been found as a newly formed mineral in soils. Here we report the recovery and identification of pure, ultrafine-grained magnetite from some UK soils that have no apparent external source of magnetite. We consider this magnetite to be of inorganic, in situ origin. The identification of magnetite formation during soil development has significance for studies of iron in soils but also has wider environmental implications. Soil-derived magnetite may contribute to the natural remanent magnetism of sediments, and act as a specific indicator of erosional events.