Heterandria is a genus of livebearing fish in family Poeciliidae, found from the southeastern United States down through Mexico into Guatemala. The atlas covers 3 species ranging from about 1.5–3 in, all sharing the poeciliid pattern of internal fertilization and live birth rather than egg-laying.
Heterandria formosa, the Least Killifish, is the genus's standout — despite the common name it is not a true killifish at all but a poeciliid, and it is famous as one of the smallest livebearing fish known, native to still, vegetated waters of the southeastern U.S. It also shows superfetation, carrying multiple broods at different stages of development simultaneously, a trait shared with only a handful of other livebearer genera. Its tiny adult size and peaceful temperament have made it a favorite for nano and native-fish setups. Heterandria attenuata (Guatemala) and H. tuxtlaensis (Mexico) are far more obscure and rarely if ever seen in the hobby.