A method for measuring the modulus of elasticity of fibers and films by sonic means is described and the results of an exploratory study of the modulus in various natural and synthetic fibers and films are given. The method is based on the equation for the propagation of longitudinal waves in elastic materials, which expresses the velocity of propagation in terms of the elastic modulus and density of the medium. The procedure involves measuring the sound velocity and calculating the modulus, using the proper density value. The modulus so calculated is characteristic of a dynamic, short period measurement, since a sound frequency of approximately 10 kilocycles per second was used. The range of moduli found was from 1×1010to 50×1010dynes per square centimeter. Correlations of moduli with other physical properties are given.