A circuit is described for the precise temperature control of the oven of a quartz-crystal frequency standard. A temperature-sensitive bridge of copper and manganin resistors is used as the amplitude control of an oscillator, power from the oscillator heating the oven. It is shown that the temperature of an isothermal oven is maintained independent of ambient temperature and depends only on the gain of the maintaining amplifier. An application of the thermostat is described in which the oven temperature of about 50° C is maintained within ±1 millideg.