Determination of the elastic and piezoelectric constants for crystals in class (3m) is complicated by the large number of independent constants and the many possible ways in which they may be combined. An experimental and analytical procedure has been developed to determine all the constants using primarily thickness‐mode measurements made on small, plate‐shaped samples of various crystallographic orientations, and results using this procedure have been obtained for lithium tantalate and lithium niobate, two recently developed synthetic crystals. The resonant and antiresonant frequency constants for thickness modes have been calculated as functions of a plate's rotation angle. Information in this form makes possible the selection of plate orientations that might be useful as resonators and transducers.