The effectiveness of ultrasound for inactivation of Escherichia coli and coliphages was quantified in this study. The results showed that sonication alone could inactivate E. coli, especially with long sonication times. A high power to volume ratio (900 W/L) provided better inactivation than a low ratio (180 W/L). Increases in turbidity and temperature were observed after application of sonication. Ultrasound provided a synergistic effect for E. coli inactivation when simultaneously applied with chlorination. No synergy was observed for sequential application of sonication and chlorination. Includes 4 references, tables, figures.