[en] An influence of a planar electric field on material coefficients of a barium titanate film has been studied. Electric-field dependences of dielectric constants, elastic and piezoelectric moduli were built for the (001)-oriented film on a cubic substrate with a misfit strain value equal to $-<!--\; ???\; -->4.0\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->3}$. It was revealed that an electromechanical coupling coefficient, k ₁₁, has a maximum at some value of the electric field, which depends on the clamping stress. The misfit strain dependence of the maximum value of the k ₁₁ coefficient was studied at strains below $-<!--\; ???\; -->1.8\times <!--\; ??\; -->{10}^{-<!--\; ???\; -->3}$ (c-phase). (letter)