The aortae of a number of vertebrates and invertebrates were tested for the presence of elastin to determine when this protein first appears phylogenetically. Using several criteria, including amino acid composition, presence of desmosines, and histological characteristics, we found that all representative vertebrate groups have elastin except the primitive jawless fishes (lampreys and hagfishes). All the invertebrates tested, representing most major phyla, were found lackin in elastin as well. Comparison of purified elastins from several vertebrate groups reveals some striking differences in their amino acid compositions and properties, including the arrangement of the elastic fibers in the aorta. The patterns of variations in amino acid composition suggest a mode of evolution which is different from the slow accumulation of point mutations observed with globular proteins.