||The Black Catbird ( Melanoptila glabrirostris ) is a poorly known endemic resident of the Yucatán Peninsula and a member of the passerine family Mimidae. I studied the social breeding system and the social and genetic mating systems of Black Catbirds in 2008 and 2009 at two sites in Quintana Roo, México: on the mainland, the Sian Ka'an Biosphere Reserve, and on Isla Cozumel. Field results yielded the first definitive report of bi-parental care without cooperative breeding in both populations, larger samples of clutch size compared to previous studies, more accurate estimates of nesting phenology, and different patterns of breeding density between populations. Genetic analyses revealed genetic monogamy in the high-density mainland population and moderate levels of extra-pair paternity in the low-density island population. This research underscores the need for more complete data sets in comparative studies of social evolution and further research on the genetic mating systems of Neotropical birds.