Almost no studies have been done with respect to major histocompatibility complex (MHC) polymorphism and sequence diversity in bats, although they account for one in five living mammalian species. We analysed MHC Class II DRB polymorphism and sequence diversity in two Mexican verpertilionid bat species, the widespread continental species Myotis velifer and the narrowly distributed (and endangered) island endemic Myotis vivesi. We find extensive DRB polymorphism in the widespread M. velifer, similar to that commonly reported in other mammals. The geographically restricted M. vivesi by contrast shows only very limited polymorphism. We conclude that M. vivesi has undergone a dramatic loss of MHC polymorphism. The significance of this inference in light of other information on population structure and genetic diversity in this species is discussed.