In primates, MHC class I family have evolved by continuous cycles of birth and death of genes leading to differential expansion of some loci in each major taxa. Humans, apes, and Old World Primates (Catarrhini) have expanded MHC-A and MHC-B loci, whereas New World Primates (Platyrrhini) have expanded MHC-G loci. Recent genomic studies in the Platyrrhini Callithrix jachuus, however, have showed several copies of MHC-B, some of which were predicted to be expressed. To inquire about the prevalence and expansion of MHC-B, and -G genes in Platyrrhini, we sequenced MHC-I cDNAs in 10 Platyrrhini species and evaluated polymorphism of MHC-G and -B in 11 individuals of Cebus albifrons. Phylogenetic analysis and genic distances showed an expansion of MHC-B with some functional copies in many species with the concomitant expansion of MHC-G previously identified. Polymorphism levels in MHC-B and MHC-G in C. albifrons were similar and both groups have been under diversifying selection at the Peptide Binding Region.