Carbonic anhydrase II electrophoretic patterns were investigated in 3113 animals belonging to 12 genera and 24 species of New World primates. Polymorphism was detected in 13 species. A total of 24 different alleles was postulated to explain the variability found; the genus Aotus showed the highest (eight) number of such alleles. Three genera of the family Callitrichidae (Callithrix, Saguinus, and Cebuella) presented five alleles that were not found among the Cebidae. Important markers at the generic level were observed in Callicebus (CA2 *6 and CA2 *12), Cebus (CA2 *10, CA2*16, and CA2 *21), and Aotus (CA2*3, CA2*4, CA2*5, CA2*9, CA2*15, CA2*17, CA2*22, CA2*23). CA2*13 seems to be the most common allele among the Cebidae; six genera of this family showed frequencies higher than 70% of it. © 1991 Plenum Publishing Corporation.