Design considerations involved in the development of low loss continuously variable resonant frequency crystal transducers are discussed. A description of such a system employing liquid mercury as a backing of continuously variable dimensions is presented. The important aspects; tight coupling of the crystal and mercury backing, and decoupling of the crystal and mercury from the supporting structure are considered in detail. Construction procedure on a unit to cover the frequency range 40 to 80 kc is indicated. Experimental results on the magnitude of the electrical input impedance as a function of frequency and mercury column length are given. A comparison of the variable resonant frequency unit used as a transducer with a series of fixed resonant frequency transducers is presented.