The complex [Mn(dbm)(2)(PY)(2)](ClO4) (dbm = anion of 1,3-diphenyl-1,3-propanedione (dibenzoylmethane), py = pyridine) was synthesized and characterized by X-ray crystallography. It has tetragonally distorted geometry with the axial positions occupied by the py ligands and the equatorial positions by the dbm ligands. This mononuclear complex of high-spin Mn(III) (3d(4), S = 2) was studied by high-frequency and -field electron paramagnetic resonance (HFEPR) both as a solid powder and in frozen dichloromethane solution. Very high quality HFEPR spectra were recorded over a wide range of frequencies. The complete dataset of resonant magnetic fields versus transition energies was analyzed using automated fitting software. This analysis yielded the following spin Hamiltonian parameters (energies in cm(-1)): D = -4.504(2), E = -0.425(1), B-4(0) = -1.8(4) x 10(-4), B-4(2) = 7(3) x 10(-4), B-4(4)= 48(4) x 10(-4), g(x) = 1.993(1), g(y) = 1.994(1), and g(z) = 1.983(1), where the B-4(n) values represent fourth-order zero-field splitting terms that are generally very difficult to extract, even from single-crystal measurements. The results here demonstrate the applicability of HFEPR at high-precision measurements, even for powder samplers. The zero-field splitting parameters determined here for [Mn(dbm)(2)(py)(2)](divided by) are placed into the context of those determined for other mononuclear complexes of Mn(III).