This fish is in the livebearer family, along with many popular aquarium fish such as guppies, mollies and swordtails. They share the same general body shape, and, like the others, have internal fertilization and bear live young. At first glance, you might mistake this the western mosquitofish for a plain-looking guppy.
This species has vertical bars across the eyes, giving it a masked appearance. Like many other livebearers, it has an upturned mouth, rounded tail fin and guppy-gray color. There is a dark edge on each scale.
Telling males from females is similar to sexing guppies: First, females are usually larger than males. Also, in adult males, the first few rays of the anal fin are greatly prolonged and grooved and serve to transfer sperm to the female. The anal fin of females looks more "finlike."