The anisotropy of the composition system MnxFe 3-xO4, 0.40≤x≤1.80, has been measured between 4.2°and 77°K by the torque method, supplementing previous measurements on this system by Penoyer and Shafer between 77°and 313°K. The anisotropy curves for 0.4≤x≤1.25 show a minimum below 110°K and appear to be the summation of a number of components of different sign and different temperature dependence - negative components which decrease in magnitude relatively slowly with increasing temperature and positive components which decrease more rapidly. Over most of the composition range studied K1 is negative at 4.2°K, but at x=0.80 the magnitude of the positive, low temperature contribution is sufficient to change the sign of K1 from negative to positive at 34°K. In the region x≤1.0, this contribution can be attributed to the presence of Fe2+ ions, but such an explanation is not applicable to the positive contributions observed in compositions for which x≥1.25 and in which there is no Fe2+. For 1.25≤x≤1.80, negative values of K1 at 4.2°K of unexpectedly large magnitude (<-3.7×105ergscm3) are observed. Unlike values at 77°K in this composition region, these values of K1 are of too large a magnitude to be attributed to the splitting of the 3d levels of Fe 3+ ions in a cubic crystalline field. © 1962 The American Institute of Physics.