A single crystal of tetragonally distorted manganese-iron spinel of composition Mn1.85Fe1.15O4 has been studied by microwave resonance in the temperature range 77-390°K. The anisotropy has tetragonal symmetry, with the c axis hard and the tetragonal a axis (corresponding to the  direction in the undistorted cubic structure) easy. The data are analyzed in terms of an anisotropy energy of the form: E A = K1α32+K2α 34+K3α14+α 24, wherein α1 and α2 are direction cosines of the magnetization referred to the tetragonal a axes, and α3 is the direction cosine of the magnetization referred to the c axis. The second-order axial component K1/M is approximately 3 kgauss at room temperature and remains relatively constant as the temperature is lowered to 160°K, below which it increases rapidly. The quantity K 3/M, which measures the anisotropy in the basal plane, is about -25 gauss at room temperature and increases to a few hundred gauss at 77°K. A temperature independent geff = 2.01 adequately describes the measurements. Linewidths vary from 250 gauss in the tetragonal  direction to 400 gauss in the c direction at room temperature and increase with decreasing temperature. © 1961 The American Institute of Physics.