MHC class II DRB variability in wild black howler monkey (Alouatta pigra), an endangered New World primate. The genes of the major histocompatibility complex (MHC) are the most important genetic component of the immune system in vertebrates. Their variability is known to influence a species' ability to recognize and respond to pathogens. Here, we present the first data of the MHC class II DRB exon 2 for the endangered black howler monkey (Alouatta pigra), one of the most northerly distributed platyrrhines. Twenty-one DRB sequences corresponding to four new lineages were identified in 44 individuals through a combination of cloning and reference strand conformational analysis. The detection of up to eight sequences per individual suggests the existence of at least four loci in the species. A relatively low DRB sequence diversity, but similar lineage and loci numbers. were found in A. pigra when compared to other platyrrhines. The reduced DRB allelic diversity in the species appears to be a consequence of drift, reflecting the colonization by its ancestors from South to Central America. Finally, the allelic diversity in the species might be enabling an adequate immune response in wild populations to cope with current pathogens, but it might entail a risk for these populations in case of the emergence of new pathogens.