We have measured initial susceptibility &khgr;0 and the dependence of anhysteretic remanent magnetization (ARM) and thermoremanent magnetization (TRM) on applied field for seven samples of magnetite, with mean grain sizes from 40 to 540 nm. TRM acquisition is nonlinear in geomagnetically relevant weak fields, contrary to the usual assumption made in paleointensity determination. Over the 40--540 nm range, &khgr;0 varies with particle shape but only weakly with particle size d and can be used to correct for varying magnetite concentrations in sediment cores. ARM is strongly size dependent over the same range and is the best parameter for monitoring grain size variations in natural samples. ARM and TRM have similar variations, as d-1 for d≤1 μm, suggesting a common source for this pseudo-single-domain (PSD) dependence on grain size. However, TRM is 10--20 times more intense than ARM in grains around 0.2 μm in size. The TRM microstate seems to be a two-domain structure, whereas the ARM microstate may be a vortex structure, which has never before been convincingly demonstrated by magnetic measurements. Independent evidence comes from theoretical fits to TRM and ARM field dependence data. In moderate and strong fields, TRM in 215--540 nm magnetites is explained by two-domain theory, but ARM is not. In smaller grains (d≤0.1 μm), a PSD theory predicts that TRM is carried by single-domain (SD) moments of entire grains but ARM resides in moments (≈1 per grain) 20--40 times weaker. These remanence levels match those of (metastable) SD and vortex states, respectively. We therefore propose that magnetite grains in the 0.1--0.5 μm size range can remain in metastable SD or two-domain states following acquisition of TRM but revert to a vortex ground state when field-cycled, for example, in ARM acquisition or alternating field demagnetization.¿ 1997 American Geophysical Union