The velocity of sound through various animal organ tissues and through living human tissues is measured, using an ultrasonic pulse method, at 1.25 and 2.5 Mc. The effect of anisotropy (fiber direction) on velocity is determined with beef muscle. Values obtained with the beam traversing the tissue perpendicularly to the long axis of the muscle bundles do not differ significantly from those found with the energy directed parallel with the muscle fibers.Velocity through living human tissues, consisting mostly of muscle, is measured by transmitting the ultrasound through various thicknesses of the arm, leg, and thigh.Specific gravities of the tissues are measured. The characteristic acoustic impedances (ρcvalues), calculated from the density and velocity data, vary between 1.5×105and 1.7×105g/cm2/sec. The imaginary component of tissue impedance is calculated and found to be negligible at the frequencies at which these measurements are made.