A low temperature thermometer based on the temperature dependence of thevF(T) of the19F NMR frequency in antiferromagnetic MnF2has been successfully operated in the 10–40 K temperature region. Novel features include the use of a MnF2crystal with a sharp19F NMR line as the temperature sensitive element in a solid state frequency‐locking autodyne circuit that operates while completely submerged in liquid H2or He. In the present design the circuit‐sample combination takes the form of a compact probe. With this device temperature readings which are reproducible to ±10−4K at 20 K have been obtained. Various applications for precision thermometry in the 10–40 K range using the device are proposed.