Spectroscopic studies have shown that, apart from a set of absorption lines due to tetravalent uranium, U :CaF2crystals exhibit two types of U3+sites. An energy level system has been determined for the sites corresponding to "isolated" ions. These sites give rise to a maser transition at 2.51 microns. The second type of site, appearing only in more highly doped crystals, was associated with the maser lines observed at 2.61 and 2.57 µ. The 2.61-µ line arises from a "four-level" system of energy states and has been made to oscillate continuously. The 2.57-µ line is a high temperature line that has only been briefly studied. The intensity of the "isolated ion" spectrum saturates as the uranium concentration is raised, while that of the "high concentration" spectrum increases roughly quadratically with increasing uranium content. Longitudinal modes have been seen in the spectra of the maser outputs at both 2.51 and 2.61 µ Both lines exhibit spiking down to helium temperatures. In a crystal exhibiting both maser lines simultaneously, maser oscillation begins at a much lower threshold for the 2.61-µ line, but the output at this wavelength saturates at high excitation levels; this saturation has not been observed in the 2.51-µ line.