The complete primary structure of theβchain from the adult hemoglobin of a baboon,Papio cynocephalus, has been determined by automated, Edman degradation of the intact chain and four fragments derived there from by specific cleavage reactions. The analysis was facilitated by application of a modified solvent system that permits unambiguous identification, by high-performance liquid chromatography, of the 17 amino acids whose phenylthiohydantoin derivatives are soluble in ethyl acetate. The sequence obtained differs from that of the humanβchain at eight sites, a degree of divergence similar to that observed when human and macaqueβchains are compared. Of the cercopithecoidβchains whose complete sequences have been determined or inferred from compositions of small peptides, that ofP. cynocephalusis most like theβchain of the gelada baboon, an observation in accord with assessment of a close phylogenetic relationship between the generaPapioandTheropithecus.