A new optical method has been developed for determining the shape of ultrasonic waves. Because of the varying index of refraction, collimated light incident on an ultrasonic wave emerges with a modulated phase. By interfering light of fixed phase with the emergent front, information about the waveform can be obtained. The theory provides for determination of ultrasonic waveform including relative phase between harmonics. The process of finding the waveform consists of simply taking the arccosine of experimental results. Comparison of experimental results with theory will be discussed. (This work was supported by the Office of Naval Research.)