Immunoglobulin Ca gene encodes a constant region of heavy-chain of IgA antibody, and the hinge region is located between CHI domain and CH2 domain. The hinge region functions to increase efficiency of antigen binding by varying distance between two antigen recognition sites. The constant region of ISA has been considered to be conservative, but we previously reported that immunoglobulin C alpha genes of great apes showed evolutionary hypervariability mainly in the hinge region (Kawamura and others, 1990). To know whether this hypervariability is also the case among Old World monkeys, we carried out PCR-SSCP analysis for macaque, baboon, and leaf monkey. The result clearly showed evolutionary hypervariability in the hinge region among Old World monkeys. Adding to this inter-species variability, intra-species variabilities were also observed in multiple species such as rhesus macaque, crab-eating macaque, Assamese macaque, and hamadryas baboon. Furthermore, polymorphism was likely shared by multiple macaque species (trans-species polymorphism). These results suggest the indispensability of hypervariability in the hinge region for these animals. Some kind of positive selection, like the overdominance at the MHC loci (Klein and others, 1993) might be responsible for this hypervariability.