The Eastern mosquitofish is a small, light-colored fish with semitransparent fins. The females usually have a black stripe near their eye area and light spots can be seen on the caudal and dorsal fins of both sexes. Due to its similar size, shape, and reproductive habits, it can easily be mistaken for a guppy. Generally, males reach 1.5 in (3.8 cm) and females 2.5 in (6.4 cm). These fish are a livebearer species, and as such, the females are larger and more rounded than the males. Pregnant females are also easily recognizable by their gravid spot; a darker area on their bellies where they hold the fry.
Mosquitofish may have a melanistic color pattern with black spots, resembling a dalmatian. This could result in its being misidentified as another species.
In its native range, Eastern mosquitofish may be confused with the Western Mosquitofish (Gambusia affinis) or the sailfin molly (Poecilia latipinna).
In eastern Australia, the female and juvenile local Pacific blue-eye (Pseudomugil signifer) are similar in appearance but have a forked tail fin.
French naturalist Charles Frédéric Girard described the species in 1859. The genus Gambusia comes from the Cuban term, "Gambusino", which means nothing. This is usually in the context of a joke or a farce. So if you are "fishing for gambusinos" then you are "catching nothing". Common names include eastern mosquitofish, plague minnow and eastern gambusia.