The effect of electrical conductivity and viscosity of the piezoelectric medium propagating plane waves has been studied. The solution of Maxwell's field equations and Newton's law of force in the general medium gives the modified elastic, piezoelectric, etc., constants necessary for determining the phase velocities of the plane waves. These modified constants are found to be complex and frequency dependent. Thus the propagation constants for the waves are complex and frequency dependent.A particular example, propagation in the 1‐direction of ammonium dihydrogen phosphate, is considered.