The magnetic structure in fine grains of magnetite has been studied theoretically using a micromagnetic model which is unconstrained except for requiring Bloch-type domain walls. Stable structures were found by second-order numerical minimization of exchange, crystalline anisotropy, and demagnetizing energies. Single- (SD), two- (2D) and three-domain (3D) structures were studied for particles at room temperature an in zero applied field. Two-domain structures with relatively large domains have very small net magnetizations because of ''skirts'' which oppose wall moments. In particles almost filled with domain walls the net magnetization can be quite high because wall moments are uncompensated. Three-domain structures always have a relatively large remanence due to unbalance domains. The critical SD-2D equilibrium size d0 was determined to be 0.084¿0.012 μm. The size range over which single-domain structures can exist is smaller than perviously predicted for perfect crystals. Experimental saturation remanence data on submicron precipitated magnetic crystals can be reasonably well fit with the present results, assuming that magnetic structures assume the lowest energy state. A technique for studying the transition mechanism between stable states is developed which demonstrates that the SD reversal mechanism is coherent rotation in particles around the critcal superparamagnetic size, while domain wall nucleation, propagation, and denucleation become favored at larger sizes. ¿ American Geophysical Union 1987