The complete amino acid sequence of the zeta chain of human embryonic hemoglobin has been determined. It differs from human alpha globin at 57 of the 141 residues and several of the replacements are at positions of structural or functional importance, particularly in relationship to the Bohr effect and high intrinsic oxygen affinity which are characteristic of embryonic hemoglobins. The zeta-globin sequence is more closely related to other mammalian embryonic alpha-like globins than to human alpha, suggesting that there have been strong selective pressures to maintain these embryo-specific globins since their emergence several hundred million years ago.