We have succeeded in direct observation of isolated vacancies in high purity silicon crystals grown by a floating zone (FZ) method using low-temperature ultrasonic measurements. The softening of elastic constants below 20 K down to 20 mK is observed in nondoped FZ silicon and B-doped FZ silicon. This softening is caused by an interaction of electric quadrupoles of triply degenerate vacancy orbital to elastic strains of ultrasonic waves. The lowtemperature elastic softening measured by ultrasonic methods verifies existence of the isolated vacancies in Pv-region distributed in pure crystal of a Czockralski ingot. The ultrasonic measurement of the low-temperature softening is a faithful probe for vacancy evaluation in high purity silicon crystals in commercial base. copyright The Electrochemical Society.