Polymorphism of erythrocytic carbonic anhydrase was studied by means of the usual technique of starch gel electrophoresis in Papio papio, Papio hamadryas, Papio cynocephalus and Papio anubis. In a sample containing both Papio cynocephalus and Papio anubis, examples of the homozygote CA-Ib/CA-Ib were found. A study of parental transmission established the CA-Ib allelic form.