We compare the magnetic properties of fine-grained magnetite produced by two newly isolated anaerobic bacteria, a magnetotactic bacterium (MV-1) and a dissimilatory iron-reducing bacterium (GS-15). Although room-temperature magnetic properties are generally different between the two microorganisms, MV-1 and GS-15 magnetites can be most easily distinguished by the temperature variation of saturation remanence obtained at liquid helium temperatures. Magnetite produced by MV-1 displays a sharp discontinuity in intensity at 100 K related to the Verwey transition. Magnetite produced by GS-15 displays a gradual decrease in intensity with temperature due to the progressive unblocking of magnetization. The differing behavior is due exclusively to different grain size distributions produced by these microorganisms. MV-1 produces magnetite with a narrow grain size distribution that is within the stable single domain size range at room temperature and below. GS-15 produces magnetite with a wide grain size distribution extending into the superparamagnetic (SPM) size range. Our results show that a substantial fraction of particles produced by GS-15 are SPM at room temperature. ¿ American Geophysical Union 1989