The complex initial permeability of a series of polycrystalline manganese-rich manganese ferrites (Mn1+xFe2-xO4) 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, Ke. Microwave resonance measurements on single crystals of the same composition give the real anisotropy constant K1. Comparison of these two experiments show that Ke~2K1. Measurement of the magnetic moment indicates that in these compounds the Mn3+ ion assumes a value of approximately two Bohr magnetons instead of the expected four Bohr magnetons.