A comparison of the acoustic response of attached and unattached individual microbubbles is presented. biSphere (TM) was insonated by 1.6 MHz ultrasound, peak negative acoustic pressures of 550 and 800 kPa, and the unprocessed radio frequency echo data collected. Comparison of the mean backscatter pressure from unattached and attached biSphere (TM) at these pressures showed no significant difference. No backscatter was detected for attached microbubbles insonated at 300 kPa. We surmised that at 550 and 800 kPa the rigid-shelled microbubble cracked on insonation, resulting in backscatter from an air microbubble in both cases. The dissolution rate of the gas was slower for attached biSphere (TM).