[en] The ultrasonic second- and third-order coupling parameters, K₂ and K₃, determined by ultrasonic second-harmonic generation, are evaluated in terms of the second- and third-order elastic moduli, for a number of crystalline systems. It is shown that K₂ and K₃ turn out simply to be the values of the elastic moduli C₁₁ and C₁₁₁, transformed into the direction of propagation. The possibility of evaluating the complete set of third-order elastic moduli from measurements of second-harmonic generation and hydrostatic pressure dependence of the sound velocity for pure mode propagation is discussed. Symmetries considered: cubic, hexagonal, rhombohedral, tetragonal, and orthorhombic