A cam tremolo device for a stringed musical instrument includes a frame secured to the instrument and a tailpiece rotatably connected with the frame and is characterized by a locking mechanism which is manually operable to prevent the tailpiece from rotating. The locking device is preferably a screw which is manually displaced by the operator relative to a threaded opening in the frame and a slot in the string attachment device connected with the tailpiece. When the screw is rotated to a locking position, the screw advances through the frame opening into the slot in the string attachment device. When the screw is rotated to a release position, the screw is retracted from the slot in the string attachment device, thereby allowing the tailpiece to rotate relative to the frame. Thus, the tailpiece is in either a fixed or floating condition, but not both at the same time.