HLA-G is a nonclassical MHC class I molecule that is primarily expressed in the placenta. To investigate whether rhesus monkeys possess an HLA-G ortholog, we cloned and sequenced MHC class I cDNAs from the rhesus placenta. We identified two rhesus MHC class I cDNAs with sequence similarity to HLA-G. Each cDNA contained premature stop codons and frameshift mutations, suggesting that it was derived from an MHC class I pseudogene. Gene trees constructed using MHC class I alleles from human and nonhuman primates revealed that the rhesus placental pseudogene alleles clustered with HLA-G orthologs from the human, chimpanzee, and gorilla. These data suggested that this rhesus MHC class I pseudogene is an HLA-G ortholog. This locus was, therefore, designated Mamu (Macaca mulatta)-G. PCR amplification of a portion of Mamu-G from the genomic DNA of five rhesus monkeys resulted in the identification of five additional Mamu-G alleles and revealed the presence of four Mamu-G alleles in one rhesus monkey, suggesting that Mamu-G had been duplicated in this individual. Furthermore, the analysis of 81 MHC class I clones isolated from a rhesus placenta cDNA library did not result in the isolation of Mamu-G cDNAs, nor the isolation of any additional HLA-G homologs, suggesting that Mamu-G was transcribed at negligible levels. Given the similarity of rhesus monkey and human placenta structure and function, these data raise interesting questions regarding the role of HLA-G in pregnancy.