Biogenic magnetites are produced through the reduction of ferric iron by both biologically induced (extracellular) and biologically controlled (intracellular) processes. With few exceptions, all are ultra-fine-grained, single-domain magnetite. Biogenic magnetites formed by magnetotactic bacteria (biologically controlled) have been shown to contribute significantly to the natural remanent magnetization of carbonates and limestones, hemipelagic and deep-sea marine sediments. The input into sediments of ultra-fine-grained magnetite produced by dissimilatory iron reducing bacteria (biologically induced) has yet to be firmly established but may be even more significant. Whether either type of authigenic biomagnetite is preserved is determined by postdepositional factors including oxidation, reduction by substitution, and dissolution. Unconsolidated and lithified sediments can be screened for putative biogenic magnetite by rock magnetic techniques. It is not generally appreciated that magnetotactic bacteria and their magnetofossils can be identified by the unusual, and in somes cases unique, morphologies, size range, and composition of the magnetite crystals. Magnetite produced by dissimilatory iron reducing bacteria have a distinctive morphology and size range, but it is currently controversial as to whether these can be distinguished from certain chemically precipitated magnetites. The presence of dissimilatory iron reducing bacteria, however, can be detected using microbiological techniques and sediments geochemistry. Biogenic magnetites are trace fossils and potentially useful environmental indicators and are considered to have significant input to the magnetization of most sediments, both modern and ancient. ¿ American Geophysical Union 1990