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Interdigital capacitive micromachined ultrasonic transducers (cMUTs) have been designed, fabricated and tested for sensing and pumping in fluidic environments. The interdigital cMUT allows one to generate unidirectional Scholte interface waves and other guided modes in microfluidic channels or to perform traditional pulse-echo measurements by changing the phase of the input signals. The transducers are surface micromachined on a silicon substrate using a CMOS compatible fabrication process which mainly uses PECVD with a maximum temperature of 300/spl deg/C. Experiments on the interdigital cMUTs show that a 5 finger pair device has 10.5 dB of directionality in generating Scholte waves at the design frequency of 10 MHz. With this configuration, interdigital cMUTs can be used as compact ultrasonic sensors. The transducers are also used for bidirectional pumping with power levels as low as 1 mW.