Abstract: An ultrasonic Doppler probe is provided for use in connection with non-invasive Doppler imaging of fluid flow within the human body. The Doppler probe can be selectively operated at more than one frequency during the course of a Doppler imaging examination thereby enhancing the resolution of the image obtained while also increasing the effective depth of the image. The probe of the present invention employs piezo-electric materials for the formation of acoustic transmitting and receiving transducers that are positioned within the probe to allow the probe to be selectively operated at a number of different frequencies spanning no more than one octave in frequency range.