The velocity and attenuation of ultrasonic waves have been determined in thallium for the case ql<1. The data are used to verify Pippard's semiclassical theory of the attenuation of sound by normal electrons. The elastic constants are given at 4.2 and 297°K. The value of the superconducting energy gap is determined for several directions of propagation and polarization of shear waves and found to be slightly anisotropic. A strain-amplitude dependence of the attenuation is found, which occurs after a minimum threshold is attained.