As the common name of this family implies, the Poeciliidae are characterised by giving birth to live young. Other family characteristics include a single soft-rayed dorsal fin and no lateral line. On male poeciliids, the anal fin has evolved into a structure known as a gonopodium that is used to transfer sperm bundles to the female fish. Poeciliidae are generally small; less than 100 mm in length, and usually the female is much larger than the male.
The Poeciliidae are native to the Americas, extending from the upper Mississippi valley through Central America down to Argentina. There may be up to 300 species in this family and many are popular with aquarists. Five species have been introduced to New Zealand but only gambusia is widespread. The other members generally require warm water habitats, and are mostly confined to thermal areas in the central North Island. The species found in New Zealand are: