Ultrasonic trapping of locomotive organisms was studied by using paramecia of ∼0.2 mm size. An ultrasonic standing wave of ∼3 MHz was generated in a sample cell to trap the paramecia. When the ultrasonic oscillation was turned on, paramecia began to swim along the nodes of the standing wave. Furthermore, when two ultrasonic waves were crossed orthogonally in a thin sample cell, paramecia were trapped at the lattice points of the nodes. The trapping efficiency increased with the ultrasonic power density. The locomotive force of the paramecia was estimated from the threshold power density for trapping. © 1997 American Institute of Physics.