Ultrasonic bursts preceding low-frequency (seismic) signals were detected on the Volcano Stromboli (Italy) for 365 over 579 seisms, by the use of three sensors for ultrasonic waves. We considered precursor signals the impulsive ultrasonic waves detected on the volcanic surface from about 5 minutes before the seism, during periods of low volcanic and acoustic activity. Since the discovered signals are precursors of seisms with the highest reliability, we can use them for an experimental study of the predictability of seisms in a geophysical model.