Heterandria formosa (known as the least killifish, dwarf topminnow, mosqu or midget livebearer) is a species of livebearing fish within the family Poeciliidae. This is the same family that includes familiar aquarium fishes such as guppies and mollies. Heterandria formosa is not as commonly kept in aquaria as these species. Despite the common name "least killifish", it belongs to the family Poeciliidae and not to one of the killifish families. H. formosa is one of the smallest fish species; the 1991 Baensch Aquarium Atlas listed it as the 7th smallest fish in the world, and as of 2006 it remains the smallest fish species found in North America.

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