In order to explain the field dependence of the steady-state electric birefringence of disk-shaped particles, a new orientation function was derived on the basis of the permanent dipole, saturable and unsaturable induced moment (PD-SUSID) mechanism. In this derivation, the covalent and/or ionic polarizability anisotropy (Delta alpha ") responsible for the unsaturable induced dipole moment (Delta alpha " E) was considered to be negative. This is the assumption that the plane of a disklike particle is oriented parallel to the direction of applied external field at infinitely high fields. Typical curves of the theoretical orientation function Phi(beta,rho(s),rho,gamma ") were graphically illustrated, and special features thereof were elaborated upon. The theoretical curves revealed some interesting characteristics associated with increasing field strengths; e.g., the theoretical orientation function plotted against the second power of field strength showed a positive maximum in certain cases, even if no permanent dipole moment was present. Comparison was made between the theoretical curves and the electric birefringence data experimentally measured up to 0.8 kV/cm for montmorillonite particles suspended in 1 x 10(-4) mol/dm(3) NaCl aqueous solution at 25 degrees C. The results indicated that the newly derived PD-SUSID orientation function for disklike particles reproduces the field strength dependence of the experimental data of the clay particle quite well.