Abstract : 27Cavitation thresholds in water were measured as a function of frequency, dissolved gas, ambient pressure, and suspended particle size. The apparatus used comprises a two-liter volume of water enclosed by a spherical glass shell driven at its radially symmetric resonance frequencies by eight multiresonant piezoelectric transducers. Large acoustic pressures can be produced, ranging from 10 bars at 27 kc/s to 200 bars at 1.16 Mc/s. The threshold data can be divided into three regions. In region A, defined by f 600 mm Hg, small air bubbles grow by rectified diffusion and stabilize at the pressure nodes. In region B, defined by f 3 bars, and P sub s 200 kc/s, P sub a > 3 bars, for any value of P sub s, transient cavities are formed, but their presence can be detected only acoustically. Experiments on cavitation at pressures larger than the threshold indicate that only a finite number of cavitation events can be produced in a given sample of water when it is isolated from contamination by airborne motes. In this way water can be strengthened by a factor of at least 8 by repeat cavitation.