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(Xiphophorus helleri), a fish of the family Poeciliidae of the order Cyprinodontidae. It is found in the fresh waters of Mexico and Guatemala. Males are up to 8 cm long, and females 12 cm. The lower part of the male’s caudal fin has the distinctive form of a sword. Swordtails are grayish olive, with a longitudinal red stripe and several shorter reddish bands parallel to it. The females, which are not as brightly colored as the males, are live-bearers. The swordtail is a popular aquarium fish. It hybridizes with the closely related platy (X. maculatus) to form black, red, lemon, mottled red, and other varieties.