The ultrasonic inactivation of Cryptosporidium oocysts was examined by high-intensity ultrasonic waves from horn type sonicator (27.5 kHz) for batch treatment and flow-through type(26.6 kHz) utilizing squeeze-film effect. After the batch treatment by the horn type sonicator, almost 80% of the oocytes were morphologically damaged after 300-sec treatment at the vibration surface amplitude of 4 um(p-p). The degree of infectivity in mice reduced to less than 1% after 60-sec treatment under the same condition. In the case of the flow-through sonicator, damage of the oocytes was observed as well but the infectivity was not correlated under the same condition. Includes 8 references, table, figures.