The dorsal fin usually has six rays, sometimes seven. There are about 30 scales along the side, which lacks a true lateral line. In life, mosquitofish are olive or dull silvery, darkest on the head and back and lightest on the belly. The scales are outlined by dark pigment and there is usually one or several rows of black spots across the caudal fin, more prominant in females. Frequently there is a wedge-shaped bar below the eye, and a very narrow dark line running along the midside from the head to the base of the caudal fin.
Pregnant females are pot-bellied and have a black spot just above the anus.
This is the only species of fish in Utah that brings forth its young alive, like the familiar guppy. Fertilization is internal, the male transferring the sperm to the genital tract of the female by means of its specialized anal fin. The breeding season occurs during the summer of Utah, or, in warm springs, it may continue throughout the year, although not as many young are born during the winter as in other seasons. The number of broods liberated by a female during one season depends chiefly on the age at which maturityis attained and varies from one to five. Increasing age brings about a descrease in fecundity because mosquitofish and other members of the family Poeciliidae , unlike most fishes, go through a period of sterile senility following the reproductive period.