Xiphophorus hellerii is a medium-sized (8–16 cm including sword) livebearer (family Poeciliidae) native to Atlantic-slope rivers and streams of Mexico, Guatemala, Belize, and Honduras. Males are instantly recognizable by the elongated, sword-like extension of the lower caudal fin rays, which can equal or exceed the body length and is typically edged in black with a yellow-green or orange interior. The body is olive-green with a red or brownish lateral stripe. Females lack the sword and are larger-bodied. Males have a gonopodium, and females are ovoviviparous. X. hellerii is found in a variety of freshwater habitats including streams, rivers, springs, and ditches, and is tolerant of moderate brackish conditions. It is omnivorous, feeding on algae, invertebrates, and plant matter. The species exhibits complex reproductive strategies: females prefer males with longer swords, supporting the sexual selection hypothesis that the sword evolved through female choice. Interestingly, the sword is an ancient trait predating the genus. X. hellerii is widely used in the aquarium hobby, where selective breeding has produced many color morphs (red, pineapple, black, lyretail). In research, hybridization with X. maculatus produces melanoma-susceptible offspring, making it central to Xiphophorus cancer genetics. The genome is sequenced and used in studies of sexual selection, speciation, and pigmentation evolution. It is invasive in many tropical regions worldwide.