Injection phase locking of reflex klystron oscillators was recently reported by Mackey in these Transactions. Two-cavity klystron oscillators may be phase locked in a similar manner. Injection locking affords a means of two-cavity oscillator stabilization that obviates the necessity for elaborate beam voltage and temperature control. Excellent frequency stability may be obtained by locking the two-cavity oscillator to a signal derived from either a crystal oscillator or a frequency standard. Fig. 1 shows a two-cavity oscillator injection locking system.