Ultrasound (US) contrast agents, used in the field of medical diagnosis, contain small microbubbles of a mean diameter of about 3 microm. The acoustic behavior of these bubbles in US field has been subject to many investigations. In this study, we propose a method to visualize the behavior of the bubbles in a 0.5-MHz US field under a microscope with a frame rate of 4 MHz. For low acoustic pressures (peak negative pressure of 0.12 MPa), the radius-time curve as measured from the optical images is in agreement with the theory. For higher acoustic pressures (peak negative pressure of 0.6 MPa), the recorded radius is significantly larger than predicted by theory and sudden change in the bubbles shapes has been noticed. The proposed method enables the study and characterization of individual bubbles and their encapsulation. It is expected that this will open new areas for quality control, US contrast imaging and US-guided drug delivery.