Abstract: A torsional ultrasonic vibrator particularly adapted for treating teeth by exposing them to ultrasonic waves in the presence of a tooth-decay retarding agent which engages the teeth. A shaft which has a central axis has fixed thereto a blade which projects from the latter axis while having a size small enough to be situated in the mouth of an individual to be treated and large enough to extend along a plurality of teeth in the mouth. A vibrator is operatively connected with this shaft for angularly oscillating the same about the above axis at an ultrasonic frequency.