Frequency equations and modal eigenfunctions have been determined for the flexural vibrations of rectangular plates of orthotropic material, for those cases which may be treated by the method of M. Lévy (1). In addition, for a broader class of boundary-value problems, an orthogonality criterion for the eigenfunctions has been established and relations for the kinetic and potential energies derived. The value of these energy functions in dealing with forced vibrations is demonstrated.