From velocity of sound measurements, obtained using Brillouin scattering and ultrasound techniques, the elastic and piezoelectric constants at room temperature have been determined on high-quality monodomain tetragonal BaTiO3 single crystals. The elastic constants are in fair agreement with those measured previously by the low-frequency equivalent circuit method. However, the electromechanical properties are significantly different. It is suggested that high frequency relaxation may have an influence on the measurements of those parameters.