Abstract: A method and apparatus for treating water such as ballast water in ships in order to eliminate aquatic organisms in the water. The water is led under pressure through a conduit into a chamber of greater cross-section than that of the conduit so that an abrupt reduction in pressure occurs. Cavitation ensues, leading to the release of dissolved gases. Ultrasonic vibration is generated and is applied to the water, exerting a pounding effect that weakens or destroys the organisms present. Other means may be used to generate further mechanical, electrical, and chemical forces in the water which attack the organisms.