2014 Values of K1, the first order anisotropy constant, have been measured by torque methods from 120 °K to 400 °K for seven crystals of gallium or aluminium substituted magnetic. The ionic distribution in these crystals has been carefully determined from other physical and chemical measurements and a preliminary analysis has been made of the results to decide the relative importance of the mechanism contributing to the anomalous variation of the anisotropy energy in magnetic. LE JOURNAL DE PHYSIQUE ET LE RADIUM TOME 20, FEVRIER-MARS 1959, Introduction. In recent years several mechanisms have been proposed to explain the observed anisotropy energies in ferrites. The relative importance of the three principal mechanisms, dipole-dipole interaction, anisotropic exchange interaction, and interaction between the individual magnetic ions and the crystalline electric field, has been investigated by Yosida and Tachiki (1957) and Wolf (1958) for Ni, Co; Fe and Mn-ferrites, although the lack of suitable experimental data on stoichiometric ferrite crystals makes any exact comparison with theory very difficult. The aim of the present work was to study the origin of the magnetocrystalline anisotropy in magnetite (Fe3O4) as this shows a rather anomalous variation with temperature. Previous workers (Bickford (1950)) have reported that the first order anisotropy constant K1 is negative at 300 OK, passes through zero at about 130,DK (the easy direction of magnetization changEs from [111] to [100]) and at 119 oR a crystallographic transition occurs, the crystal symmetry reducing from cubic to orthorhombic owing to the long range ordering of ferric and ferrous ions on the octahedral lattice sites, e.g. Verwey and Haayman (1941). Bickford (1957) has shown from anisotropy measurements on cobalt substituted magnetite that the change in sign of K1 in magnetite is not due to the presence of cobalt impurity and one might therefore assume the change is caused by sbort range ordering of Fe2+ and Fe3+ ions at temperatures above the transition point. However Rado (1958) has measured the anisotropy constants of some mixed crystals of magnesium ferrite and magnetite and suggests that .the Fe3+ ions give contributions of opposite sign to the anisotropy depending on whether they occupy the tetrahedral or the octahedral sites, thus providing support for the crystal field theory which predicts similar results according to Yosida and Tachiki (1957). In this paper results are presented of anisotropy measurements made on substituted single crystals of magnetite of nominale composition Ga,,Fe3’o4 (where x = .08, .14, .27, .36) and AI.Fea-v04 (where y = .10, .21). Substitutions of gallium and aluminium were chosen because of their tendency to