One of the authors (L.N.) describes the mechanism of magnetization of a ferromagnetic single crystal:the elementary domains can be classified in several groups, called “phases” each having a certain direction ofthe spontaneous magnetization. The variation of the total magnetization takes place in various “modes,”corresponding to the number of coexisting phases in a given field. The calculation of the magnetizationcurve in these various modes explains the experimental results for single crystals of iron, pyrrhotine andmagnetoplumbite.The results of measurements at room temperature on this latter compound,6Fe2O3-PbO, are treatedin greater detail. The crystal is uniaxial; for different angles of the external field with thecaxis, the magnetizationfirst increases proportionally to the field, followed by a gradual approach to saturation (exceptfor the angles 0 and 90°). The anisotropy constantK1is first determined (K2is negligible). The two partsof the curve can be interpreted by two “modes,” with respectively, two and one “phases.”The variation of the magnetization of polycrystalline6Fe2O3-BaOsamples can be explained by therotation mechanism only; the crystallites are too small in size to be divided into phases. The experimentalcurves can be explained by taking into account the interactions between crystallites in calculating the lawof approach to saturation.