Large single crystals of benzene C6H6 were aligned by an optical method. The aligned crystals were pulsed with ultrasonic waves at a frequency of approximately 10 megacycles per second using quartz crystal transducers. The velocity of the ultrasonic waves was measured by comparing the echo transit time with that of simultaneously generated waves moving through a variable length path in a medium in which the velocity of sound was known precisely. Velocities of longitudinal and transverse waves along each of the principal axes of the orthorhombic crystal were used to compute the main diagonal terms of the C sub ij matrix. The off-diagonal terms were then determined by velocity measurements along arbitrarily selected directions. Velocity measurements were made and the elastic constants computed over the temperature range 170 K to 250 K. Absorption in the crystal was measured over the same temperature range for each of the directions used for velocity measurements. The absorption was determined to be aelotropic.