Angiotensin I (ANG I) of the American alligator Alligator mississippiensis was isolated from incubates of homologous plasma and kidney extract, and its amino acid sequence was determined as H-Asp-Arg-Val-Tyr-Val-His-Pro-Phe-Ala-Leu-OH. The presence of strongly hydrophobic alanine at position 9 is unusual among ANGs I sequenced to date. Since alanine can be converted to serine by a one-point mutation of the triplet nucleotides, the phylogenetic proximity of the alligator to birds, whose ANG I has serine at position 9, is confirmed by the chemical evolution of the hormone structure. The alligator [Asp1,Val5,Ala9] ANG I increased arterial blood pressure of conscious alligators, but it was no more potent than avian [Asp1,Val5,Ser9] ANG I. The alligator ANG I was also equipotent to the homologous ANG I in the quail and rat. It is concluded that substitutions at position 9 have little influence on the vasopressor activity of ANG I.