A new type of constant-frequency oscillator of very high stability is presented. The frequency-controlling resonant element is used as one arm of a Wheatstone resistance bridge. Kept in balance automatically by a thermally controlled arm, this bridge provides constancy of output amplitude, purity of wave form, and stabilization against fluctuations in power supply or changes in circuit elements. A simple one-tube circuit has operated consistently with no short-time frequency variations greater than ± 2 parts in 108. Convenient means are provided for making precision adjustments over a narrow range of frequencies to compensate for longtime aging effects. Description of the circuit is followed by a brief linear analysis and an account of experimental results. Operating records are given for a 100-kilocycle oscillator.