Amino acid sequences of fibrinopeptides A and B from the macaque, Macaca fuscata (Japanese monkey) and the guenon, Erythrocebus patas (patas monkey) were established. Fibrinopeptides A of the monkeys had a sequence identical with those of baboons:Ala-A_??_p-Thr-Gly-Glu-Gly-A_??_p-Phe-Leu-Ala-Glu-G_??_y-Gly-Gly-Val-A_??_g. Fibrinopeptides B were 9-residue, “short, ” peptides with the sequences Asn-Glu-Glu-S_??_r-Leu-Phe-S_??_r-Gly-Arg for M. fuscata and Asn-Glu-Glu-V_??_l-Leu-Phe-G_??_y-Gly-Arg for E. patas. The sequence of the B peptide of M. fuscata differed from that of a close-related species, M. mulatta (rhesus monkey), at a single site, L_??_u (M. f.)→P_??_o (M. m.). A single replacement between the B peptides of E. patas and Cercocebus aethiops (green monkey), V_??_l (E. p.)→G_??_y (C. a.), was detected. A phylogenic relationship of macaques, guenons, and baboons, named Cercopithecinae (Old World monkey), was deduced from the sequence data. A selective rather than random amino acid replacement was observed in the B peptides of these Old World monkeys, suggesting a restricted mutation of their fibrinopeptides during primate evolution.