Parallel field excitation of piezoelectric plates has been analyzed with a matrix method, and the admittance of the plate is derived. This formulation affords the definition of coupling factor to each of three vibrational modes, which is shown to be modified by the antifield effect due to the induced charges on major surfaces of the plate in case of anisotropic crystals. In contrast to the perpendicular field excitation case, the resonant frequencies in this case are odd multiples of the fundamentals and the antiresonant frequencies are determined as solutions of a transcendental equation. The validity of the derived equations is confirmed by comparing measured and calculated frequency constants of lithium tantalate single crystal.