Ferromagnetic resonance line widths in dense polycrystalline and single crystal specimens, the magnetic moment and the anisotropy field (K1/Ms) have been measured as a function of temperature. Between room temperature and the Curie point the excess resonance line width, which is given by &Dgr;H(excess)‐&Dgr;H(polycrystal)—&Dgr;H(single crystal), is found to have the same temperature dependence as the saturation magnetization. This result is obtained even though the polycrystalline density (>98%) is sufficiently high to be in the range where the excess line width has usually been attributed to anisotropy broadening. In the region below room temperature &Dgr;H(excess) becomes dependent on the anisotropy field. An approximate criterion, that is suggested by the data, to determine whether the anisotropy field or the magnetic moment is responsible for the broadening is the ratioK1/Ms2≷0.25. The compositions measured were Mn1.24Fe1.76O4and Mn1.49Fe1.51O4. The polycrystallinegfactor (gp) is greater than the single crystalgfactor (gs) and bothgfactors are functions of the temperature. The shift in the polycrystalline resonance peak is discussed with reference to the Standley Stevens theory and limited agreement is found. The anisotropy constant,K1, has been found to vary as the fifth power ofMsfor these ferrite compositions.