The effectiveness of ultrasound for inactivation of Escherichia coli and MS2 coliphage was quantified. The results showed that sonication alone could inactivate both organisms to non- detectable levels, especially with long sonication times. A high power to volume ratio (900 W/L) provided better inactivation than a low ratio (180 W/L). Inactivation resulted predominantly from temperature effects as temperature increased to greater than 70C in less than 10 minutes at 900 W/L. Sequential application of sonication followed by chlorination did not show synergy; rather, inactivation by the two disinfectants was less than additive. Simultaneous application of sonication and chlorination showed significant synergism. Even with short contact times, when temperature increases were not a factor, 4 to 5 log10 inactivation of E. coli was achieved by simultaneous disinfection, compared to less than 2 log10 inactivation that would be predicted if the effects were only additive. Similar trends were observed in MS2 inactivation experiments. Includes 8 references, tables, figures.