A compilation, from the original papers, is presented of the data on the acoustic properties (i.e., velocity and attenuation) of mammalian tissues in the megahertz range of frequencies used in medical ultrasonics. The actual values reported have been quoted to present a clear picture of the current state of knowledge in order to enable it to be assessed and to enable future workers to compare their own results directly. In the 20 years since the last such compilation, the progress in estimating the relative contributions of the animal species, the condition of the tissues, the temperature and frequency at which measurements were made, and the method of measurement to the observed variation has been small. It is hoped that this compilation together with summarizing tables will both be of practical use to the worker in the field and also encourage the establishment of a stronger body of fundamental information for the application of ultrasonics in biology and medicine.