This paper shows the applications ultrasonic through-transmission technique to determine the elastic constants of two polymernatural fiber composite materials with potential industrial application and economic and environmental advantages.The transversely isotropic coconut-epoxy and fique-epoxy samples were analyzed using an experimental setup which allows the sample to be rotated with respect to transducers faces and measures the time-of-flight at different angles of incidence.Then, the elastic properties of the material were obtained by fitting the experimental data to the Christoffel equation.Results show a good agreement between the measured elastic constants and the values predicted by an analytical model.The velocities as a function of the incidence angle are reported and the effect of the natural fiber on the stiffness of the composite is discussed.