For five groups of aortic specimens (normals and with different degrees of atherosclerosis) the ultrasonic backscatter coefficient was measured as a function of frequency in the range 4-15 MHz. The results of the study are related to two classes of structure, connective and fatty tissue (whose relative amount in the arterial wall is related to the different stages of atherosclerosis), as the main determinants of the scattering from aorta. The structure of connective tissue in the aorta produces a power law frequency dependence of the backscatter coefficient typical of small scale structures (Ka much less than 1). Fatty tissue introduces a frequency dependence of the backscatter coefficient typical of structures of intermediate scale (Ka approximately equal to 1). Biochemical composition and structure of normal and atherosclerotic aorta therefore, specifically affect the employed acoustical parameter.