The trigonal pyramidal complex [Mn4O3Cl(O2CCH3)(3)(dbm)(3)], where dbm(-) is the monoanion of dibenzoylmethane, functions as a single-molecule magnet. High-field EPR data are presented for an oriented microcrystalline sample to characterize the electronic structure of the (MnMn3III)-Mn-IV complex. These data show that the complex has a S = 9/2 ground state, experiencing axial zero-field splitting (D (S) over cap(z)(2)) With D = -0.53 cm(-1) and a quartic zero-field splitting (B-4(0) (O) over cap(4)(0))With B-4(0) = -7.3 x 10(-5) cm(-1). Magnetization versus external magnetic field data were collected for an oriented single crystal in the 0.426-2.21 K range. At temperatures below 0.90 K hysteresis is seen. Steps are seen on each hysteresis loop. This is clear evidence that each (MnMn3III)-Mn-IV complex functions as a single-molecule magnet that is magnetizable. Furthermore, the steps on the hysteresis loops are due to resonant magnetization quantum mechanical tunneling. In response to an external field each molecule reverses its direction of magnetization not only by being thermally activated over a potential-energy barrier, but by the magnetization tunneling through the barrier. Additional evidence for resonant magnetization tunneling was found in the change in the temperature at which the out-of-phase ac magnetic susceptibility is observed as a function of Bn external de field. The results of magnetization relaxation experiments carried out in the 0.394-0.700 K range are presented. These data are combined with the ac susceptibility data taken at higher temperatures to give an Arrhenius plot of the logarithm of the magnetization relaxation rate versus inverse absolute temperature. The temperature-dependent part of this plot gives an activation barrier of 11.8 K. Below 0.6 K the relaxation rate is independent of temperature with a rate of 3.2 x 10(-2) s(-1). This S = 9/2 single-molecule magnet exhibits a tunneling of its direction of magnetization at a rate of 3.2 x 10(-2) s(-1) in the 0.394-0.600 K range. Thus, resonant magnetization tunneling is seen for a half-integer-spin (S = 9/2) ground-state magnet in the absence of an external magnetic field. The transverse component of the small magnetic field from the nuclear spins is probably the origin of this tunneling.