Velocity and attenuation of both transverse and longitudinal ultrasonic waves are measured in a buna‐N vulcanizate at 2, 5, and 10 megacycles, over a range of temperatures. The method involves use of solid transmission media to conduct pulsed signals into a thin flat specimen. Values of the bulk modulus and of the complex shear modulus are computed from the measurements. It is found that longitudinal‐wave losses in this particular synthetic rubber are substantially accounted for by the relaxation phenomena measured in shear, but this conclusion is subject to considerable quantitative uncertainty arising from experimental errors estimated to be as large as 5 percent for longitudinal‐wave acoustic data and 10 percent for transverse‐wave acoustic data. It is found that below approximately −25°C the shear‐modulus loss tangent becomes relatively independent of temperature and frequency in the present range.