A new scheme for locking an acoustic resonance of an acousto‐optic modulator to the exciting rf field applied for laser mode locking is presented. The resonance of the mode‐locking quartz prism is temperature‐tuned to match the synthesizer frequency. Deviation from perfect matching is sensed by observing the relative phase of the rf wave reflected at the device, and the error signal is fed back by a servo loop adjusting the heat delivered by a heating transistor to maintain the temperature required for exact resonance. The performance of the resonance‐tracking loop was checked on several different resonances of the modulator (used in an argon‐ion laser), because switching to another resonance is a simple and attractive way for changing the laser pulse repetition rate (or mode spacing) without replacing the mode‐locker unit. Excellent loop response was found in all cases. The new tracking principle provides a widely extended tracking range, improved tracking accuracy and laser stability, and simplifies the laser adjustment.