Abstract It is shown that a wave which is nearly polarized in the shear horizontal (SH) direction can exist in thin plates of arbitrary piezoelectric materials. The wave is actually a Lamb wave (the A 1 Lamb wave mode) in which all three components of particle displacement are present. The shear horizontal component, however, becomes dominant if the ratio h λ ( h = plate thickness, λ = acoustic wavelength) becomes less than about 0.5. Attractive properties of this quasi-SH wave include: (a) phase velocity nearly constant for all values of h λ ; (b) ability to propagate in contact with a liquid medium; and (c) very high value of K 2, the electromechanical coupling coefficient. These properties make the wave attractive for use in a variety of sensor and signal processing applications. Experimental measurements of the phase velocity and coupling coefficient of the mode are found to be in fair agreement with theoretical calculations. An example of sensor applications is illustrated by using the wave to measure conductivity of liquids (aqueous KCl solution). The frequency of a 12 MHz quasi-SH mode oscillator fabricated on a 0.48 λ thick Z- X lithium niobate plate is found to vary by more than 80 kHz for a variation in KCl concentration from 0 to 0.15%.