The mechanical and ultrasonic parameters of kidney stones have been measured in vitro. The techniques used for these measurements are briefly described and the data are presented. It was found that the mechanical crushing strength correlates with the acoustic velocity in the calculi, which in turn is a function of the stones' chemical deposition. It was shown that the crushing strength increased exponentially with velocity in the range from approximately 22 kg/mm2 to approximately 100 kg/mm2. The corresponding ultrasonic velocity in the stones varied between approximately 2,200 m/sec to about 3,000 m/sec. It is suggested that these data could be used to determine the desirable characteristics of a piezoelectric lithotripter.