The magnetostratigraphy of deep-sea sediment cores has proven to be a powerful and high-resolution method of dating and comparing sedimentary records. In contrast to this widespread application, relatively few attempts have been made to analyse and identify the mineralogy, grain size and origin of the carrier of the remanent magnetization^1,2. It is widely assumed that dominant ferrimagnetic minerals in deep-sea sediments are of lithogenic origin; that is, that they derive from oceanic or continental detritus, volcanic ashfalls, micrometeorites, hydrothermal precipitation or chemical mineralization processes. A very different view was taken by Kirschvink and Lowenstam³, who suggested that fossil remnants of magnetite-mineralizing organisms, in particular magnetotactic bacteria, might play an important part in the magnetization of sediments deposited in certain detritus-poor aquatic systems. We have used rock-magnetic diagnostic methods to characterize the magnetic phases in deep-sea sediments from the South Atlantic; these phases were then extracted and studied with the electron microscope. Our results indicate that fossil bacterial magnetite is the main carrier of the natural remanent magnetization in sediments ranging in age from Quarternary to Eocene. Different types of bacterial magnetosomes were detected, most of them occurring in chains of identical particles, and nearly all of them falling into the theoretically determined⁴ size range of single-domain magnetitite.