Precise measurements of the temperature dependences of the53Cr,79Br, and81Br nuclear resonance frequencies in ferromagnetic chromium tribromide have been carried out between 4.2°K and the Curie temperature (32.56°K). The resonances are found to be sensitive and reproducible indicators of temperature. Several of the resonances can be used for thermometers with millidegree sensitivity in the range 2°–25°K. This range includes the 4°–14°K range between the hydrogen and helium vapor‐pressure curves, where a reliable secondary standard is needed.