Paramagnetic resonance absorption measurements of uranium in various symmetries in CaF2 and SrF2 crystals are given. It is concluded that the centers seen in trigonal symmetry are U4+ ions. This conclusion is based on the observed direct correlation between the trigonal center resonance and one of two components which, generally speaking, together constitute the optical absorption spectra of these crystals. This spectral component is isolated and shown to agree with the published optical absorption spectra of U4+ compounds. Also, it is shown that the observed asymmetry of the trigonal center resonance and its zero g value characterize the ground state (f2, H43), in trigonal symmetry and cannot be properties of the state (f3, I924). A model is proposed for the U4+ center. © 1962 The American Physical Society.