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Some of the factors which affect domain structures in magnetic materials with high magnetocrystalline anisotropy include crystal perfection, nonmagnetic or weakly magnetic inclusions, and the nature of grain boundaries. In whisker-like barium ferrite crystals about 3 μm in thickness straight (180°) domain walls were observed in the basal plane. Reverse domains did not nucleate at fields of over -5000 Oe after first saturating the crystals in a positive magnetic field. In less perfect crystal platelets of about the same thickness the domain structures form in what may be described as a convoluted pattern, and after saturating in a positive magnetic field, reverse domains nucleate in positive fields of from +100 to +1000 Oe. An analysis of the convoluted patterns has shown disclination structures of several types. The influence of internal defects as well as the effect of surfaces, nonmagnetic inclusions, and grain boundaries on domain structures in magnetic materials with high crystalline anisotropy are described and discussed.