A simple design for continuous filtration of cells and other particles in a one MHz ultrasonic standing wave field is described. Using suspensions of yeast cells, filtering efficiencies of greater than 99% have been achieved at a flow rate of 4.8 ml min(-1). At this flow rate, efficiencies exceeding 95% were reached for all concentrations between 1.5 x 10(7) and 3.9 x 10(9) cells ml(-1). Yeast suspensions were concentrated up to 1.7 x 10(10) cells ml(-1) (equivalent to 9/10 of the packed cell pellet formed in a centrifuge tube at 2,000 g for 10 min). Concentrations from 1.4 x 10(6) to 7.2 x 10(9) cells ml(-1), flow rates between 2.4-30 ml min(-1), and a range of transducer voltages were used to characterize the filters efficiency. The system is compact with a dead space of less than 10 ml. The filtering action is direct result of an interaction between the particles in suspension and the ultrasound. Consequently, there are no complex structures which could become blocked. The system has no moving mechanical parts, giving a robust filter which works over a wide range of conditions. The potential for using the current system as a component in a fluid processing circuit is discussed.