Gambusia is a large genus of viviparous fish in the family Poeciliidae (order Cyprinodontiformes). Gambusia contains over 40 species, most of which are principally found in freshwater habitats, though some species may also be found in brackish or saltwater habitats. The type species is the Cuban gambusia, G. punctata. The greatest species richness is in Mexico, Texas, and the Greater Antilles, but species are also found elsewhere in the eastern and southern United States, the Bahamas, Central America, and Colombia. Gambusia species are often called topminnows, or simply gambusias; they are also known as mosquitofish, which, however, refers more specifically to two species, G. affinis and G. holbrooki, which are often introduced into ponds to eat mosquito larvae.[3][4] As a consequence, they have been introduced widely outside their native range, and frequently become invasive, threatening local species.[5] They are only occasionally kept in aquariums, due to their relative lack of color and the highly aggressive nature of the aforementioned mosquitofish species.
Nine species are listed as vulnerable in the IUCN Red List; one, the widemouth gambusia, G. eurystoma, is critically endangered; and two, the Amistad gambusia, G. amistadensis, and the San Marcos gambusia, G. georgei, are already extinct.