Approximately equidimensional magnetite crystals, with mean sizes of 215, 390, and 540 nm, respectively, have been producing hematite crystals. Isothermal magnetic hysteresis properties show a clear progression toward multidomain-like behavior as the mean grain size increases. Saturation remanences Mrs are only 5--10% of saturation magnetization Ms, coercive forces HC are low (5.5--8 mT), and both Mrs and HC have grain-size dependences compatible with those previously established for smaller and larger hydrothermally produced magnetites. Coercivities during remanence acquisition are greater than those measured during demagnetization. The difference between acquisition and destructive fields increases in the larger grains as a result of the increasing importane of the internal demagnetizing field. The low-temperature transition is well expressed in the Mrs and HC data of the 540-nm sample but is more subdued for smaller grains. Magnetostrictive, magnetocrystalline, and magnetostatic mechanisms in turn govern coercivity as the temperature rises. Remanene and coercivity ratios, Mrs/ Ms and HR/HC, are almost temperature independent up to 500 ¿C, indicating that domain wall configurations resulting from saturating fields are about the same at any temperature. A thermal fluctuation analysis of high-temperature coercive force data suggests that regions 200--250 nm in size are thermally activated as a unit in grains of all sizes; these are likelly domain walls. Apparent emagnetizing factors calculated from both low- and high-empeature data are consistent with a mixture of two-domain (2D) and three-domain (3D) grains in all samples. However, theories of remanence in conventional 2D and 3D grains or in mixtures of 2D, 3D, and metastable single-domain grains do not explain the data in a satisfying way. ¿ American Geophysical Union 1990