High-field, single-crystal EPR spectroscopy on a tetragonal bisdiselenazolyl ferromagnet has provided evidence for the presence of easy-axis magnetic anisotropy, with the crystallographic c axis as the easy axis and the oh plane as the hard plane. The observation of a zero-field gap in the resonance frequency is interpreted in terms of an anisotropy field several orders of magnitude larger than that observed in light-heteroatom, nonmetallic ferromagnets and comparable (on a per-site basis) to that observed in hexagonal close packed cobalt. The results indicate that large spin orbit-induced magnetic anisotropies, typically associated with 3d-orbital-based ferromagnets, can also be found in heavy p-block radicals, suggesting that there may be major opportunities for the development of heavy p-block organic magnetic materials.