The complex initial permeability of a series of polycrystalline manganese-rich manganese ferrites (${\mathrm{Mn}}_{1+x}{\mathrm{Fe}}_{2\ensuremath{-}x}{\mathrm{O}}_{4}$) has been measured between 20 Mc/sec and 2000 Mc/sec. Three resonances are detected; the two lower-frequency resonances, which occur below 200 Mc/sec, are associated with domain wall motion. The higher-frequency resonance, which varies between 300 and 600 Mc/sec depending on the composition, allows the calculation of an effective polycrystalline anisotropy constant, ${K}_{e}$. Microwave resonance measurements on single crystals of the same composition give the real anisotropy constant ${K}_{1}$. Comparison of these two experiments show that ${K}_{e}\ensuremath{\approx}2{K}_{1}$. Measurement of the magnetic moment indicates that in these compounds the ${\mathrm{Mn}}^{3+}$ ion assumes a value of approximately two Bohr magnetons instead of the expected four Bohr magnetons.