Abstract : We investigated the disinfection of several bacteria relevant to ship's graywater and sewage, and bacteria and zooplankton relevant to ship's ballast water, using high power ultrasound (HPU) driven by the magnetostrictive material, TERFENOL-D. TERFENOL-D is capable of generating high power ultrasound more efficiently than previously existing technologies. Initial experiments were carried out at the beaker scale. Results from these experiments were used to make recommendations for area of focus in pilot-scale experiments. Beaker-scale results were also used to specify the flow parameters of a scaled-up treatment system. Etrema Products,Inc. applied these flow parameters in designing and building a pilot-scale (50-gpm) treatment system based on a 3 kW TERFENOL-D transducer and cascade horn. Experiments at the pilot scale investigated the effects of contact time or flow rate, ultrasonic intensity or energy density, and pressure on the effectiveness of HPU for treating bacteria and zooplankton. Contact times and energy densities appear feasible for treatment of zooplankton such as Artemia, in the context of flow rate required for the ballast water application; however, contact times and energy densities were orders of magnitude higher for bacteria such as E. coli.