Pronounced temperature‐dependent transfer of excitation from the Er3+laser pump levels to lower levels of Er3+was observed in CaF2:ErF3single crystals at ErF3concentrations exceeding 12.5 wt %. Transfer of excitation from Er3+to Tm3+occurred when TmF3was included as a second active constituent of the crystals. Stimulated emission of the4I11/2→4I13/2transition of Er3+at 2.69 &mgr; at 298°K resulted independently of Tm3+and as a consequence of concentration quenching of emission of Er3+from levels above4I11/2. At 100°K reduced transfer of excitation to4I11/2of Er3+occurred due to radiative emission from levels above4I11/2, and laser emission of Tm3+was detected at 1.86 &mgr;.