Gambusia is a large genus of fish in 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, Gambusia punctata). Gambusia species are often called topminnows or simply gambusias; they are also known as mosquitofish, which, however, refers more specifically to one species, G. affinis. Gambusia can be introduced into ponds to eat mosquito larvae. They are very important in aquarium trade, desired for small size, ease of breeding, and charming gracefulness. They are viviparous—they have live young.

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.

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