High-field electron paramagnetic resonance spectroscopy shows that the structurally distorted Mn-III ion in Na-5[Mn(l-tart)(2)]center dot 12H(2)O (1; l-tart = L-tartrate) has a significant negative axial zero-field splitting and a small rhombic anisotropy (similar to 1% of D). Alternating-current magnetic susceptibility measurements demonstrate that 1, which contains isolated Mn-III centers, displays slow relaxation of its magnetization under an applied direct-current magnetic field.