Determining the threshold of fragmentation of ultrasound contrast agents is important for both imaging and therapeutic ultrasound applications. We detected acoustic emissions from Optison™ microbubbles that were insonified by pulses of ultrasound. Our observations suggest that when the microbubbles rupture, daughter bubbles are created which subsequently grow and then collapse on a time-scale of 1-5 μs. The emission from the "rebound" collapse occurs after the end of the excitation pulse and we used the presence of this signal to determine the thresholds for the shell rupture. These shell-disruption thresholds were found to increase with frequency and decrease with pulse length.