A train of 1‐MHz pulses can generate maxima of cavitation activity [V. Ciaravino, H. G. Flynn, and M. W. Miller, Ultrasound Med. Biol.7, 159–166 (1981)] at pulse lengths of 6 and 60 ms and at pressure amplitudes,PA, between 5.4 and 9.4 bars (or intensities between 10 and 30 W/cm2). Generation of maxima atPAbetween these limits on pressure amplitude implies that the increase in cavitation activity originates from gas nuclei with radii lying in a critical size range centered at about 0.08 μm. The mechanism proposed for this phenomenon suggests that nuclei in this critical range are unstabilized nuclei generated in one pulse and surviving to the next with an appreciable fraction of the survivors lying in the critical range. Transient cavities that grow from such small nuclei are shown to behave as isolated mechanical systems that on reaching maximum size collapse as imploding spheres. The maximum pressures reached in such imploding cavities would then approximate those calculated for the spherical collapse of cavities. The occurrence of the observed maxima is ascribed to the spherical collapse of transient cavities.