The magnetic properties of low field susceptibility (&khgr;), frequency dependent susceptibility (&khgr;fd), and susceptibility of anhysteretic remanent magnetization (&khgr;ARM) are used to define sets of natural soil and sediment samples within which, the the criteria of King et al. (1982) and Maher (1988), the modal ferrimagnetic grain diameter is less than ~0.07--0.1 μm. Within this sample set, two distinct groups with low and high values, respectively, for the quotients &khgr;ARM/&khgr; and &khgr;arm/&khgr;fd are defined. The first group includes sediment samples from sites where published studies propose a detrital origin for the fine-grained ferrimagnetic content. Where catchment samples are available for comparison, they fall within the same envelope, as do clays from palaeosol samples within Chinese loess. This envelope of low quotient values also overlaps with the values for the fine grained synthetic magnetite samples within Maher's (1988) New MT series. The high quotient envelope of values includes sediments from the Adriatic Sea and clays from saltmarsh and shallow water marine sediments in NW Britain. In these cases, no catchment source is postulated for the fine-grained magnetite. The size range of magnetite in this set appears to be almost exclusively stable single domain (SSD), and a bacterial origin seems likely. Sample sets from estuarine environments between river inflow sites and the open sea show intermediate values which, in the case of the Potomac, are ordered by distance down river. Although at this stage, magnetic measurements alone cannot discriminate between bacterial and fine-grained detrital ferrimagnets in sediments, they hold out some promise of doing so provided the distinctions proposed here can be substantiated by transmission electron microscopy. ¿ American Geophysical Union 1994