Alpha-globin genes in crab-eating macaques were found to be triplicated at high frequencies according to restriction-enzyme comparisons. The frequencies of triplicated alpha-globin genes in macaques originally from Malaysia and Indonesia were 0.432 and 0.275, respectively, while no triplication was found in individuals from either the Philippines or northern and central Thailand. Quadruplicated alpha-globin genes were also observed, at frequencies of 0.045 (Malaysia), 0.075 (Indonesia), and 0.021 (the Philippines). A single locus was detected in only one of 40 chromosomes from Indonesia (frequency 0.025).