The green swordtail (Xiphophorus hellerii) is a species of freshwater/brackish fish in family Poeciliidae of order Cyprinodontiformes. A live-bearer, it is closely related to the southern platyfish or "platy" (X. maculatus) and can crossbreed with it. It is native to an area of North and Central America stretching from Veracruz, Mexico, to northwestern Honduras.
The male green swordtail grows to a maximum overall length of 14 cm (5.5 in) and the female to 16 cm (6.3 in). The name "swordtail" is derived from the elongated lower lobe of the male's caudal fin (tailfin). Sexual dimorphism is moderate, with the female being larger than the male, but lacking the "sword". The wild form is olive green in color, with a red or brown lateral stripe and speckles on the dorsal and, sometimes, caudal fins. The male's "sword" is yellow, edged in black below. Captive breeding has produced many color varieties, including black, red, and many patterns thereof, for the aquarium hobby.
The green swordtail prefers swift-flowing, heavily-vegetated rivers and streams, but is also found in warm springs and canals. Omnivorous, its diet includes both plants and small crustaceans, insects, and annelid worms.
X. hellerii has become a nuisance pest as an introduced species in a number of countries. It has caused ecological damage because of its ability to rapidly reproduce in high numbers. Feral populations have established themselves in southern Africa, including Natal, Hawaii, Madagascar and eastern Transvaal in South Africa and Otjikoto Lake in Namibia. Significant populations have also established themselves along the east coast of Australia.
One of the most popular tropical aquarium fish, the green swordtail has been bred into various hybrid forms for the aquarium hobby due to its hardiness and suitability for community tanks.
It is often designated X. helleri (with one "i"), but authorities consider this an orthographic error and the spelling with two "i"s to be the valid specific epithet. Due to interbreeding with the southern platyfish or "platy", most swordtails in aquaria are hybrids to some degree.
The males' beautiful, elongated caudal fins have been found to significantly affect their chances at mating. The presence of a well-endowed male spurs the maturity of females, while it inhibits the maturity of juvenile males in the vicinity of the well-endowed male.