Electric birefringence (EBF) was studied for solutions of a poly(butyl isocyanate) high-molecular-mass fraction (M_Dη = 770 × 10³) in tetrachloromethane in strong fields. The deviation from the linear dependence of EBF on the squared field strength was revealed. The experimental dependence was compared with the theoretical relationships calculated for the rodlike, freely jointed, and rotational-isomer models of a chain. It was shown that the values of the statistical segment length and the dipole moment of the monomer unit obtained for the rotational isomeric state model of a polymer chain on the tetrahedral lattice are consistent with known experimental data. The experimental results obtained confirm the theoretical conclusion that the orientation of the rotational-isomer polymer chain with a constant longitudinal dipole moment is equivalent to that of the 'effective' freely jointed chain with the field-dependent length of the rigid segment. The length of the effective segment is close to the average length of the regular trans sequence of a polymer chain on the tetrahedral lattice.