Fish born early in the spring may reproduce later in the summer and fall. Those born late in the reproductive season overwinter before reproducing (Krumholz 1948). In southcentral Texas, young may be collected from March to October with a peak in abundance in April (Davis 1978). In some constant temperature springs, these fish cease reproduction in winter (Brown and Fox 1966, Davis 1978). However, some populations from thermal habitats (such as cooling ponds and lakes) reproduce year-round (Ferens and Murphy 1974, Bennett and Goodyear 1978). At the Savannah River Power Plant site, South Carolina, fish reproduce throughout the winter although at much reduced brood sizes (Meffe, pers. comm., cited in Constantz 1989). These same workers found that the percentage of reproductively active females increased with increasing water temperature.

Mosquitofish have internal fertilization and are ovoviviparous (Sublette et al. 1990). Females can store sperm from one copulation and fertilize several broods sequentially (Krumholz 1948). After a gestational period of 21 to 28 days, the young are born alive at a size of approximately eight to nine mm total length (Krumholz 1948). Larger females produce more offspring (Krumholz 1948). Brood sizes of one to 315 young have been reported (Barney and Anson 1921, Moyle 1976). Females annually have four to five broods (Krumholz 1948). Sex ratios are 1:1 at birth, but in older cohorts, the number of males declines relative to the number of females (Krumholz 1948). Under optimal conditions females can become gravid at 6 weeks of age, produce 2-3 broods in first summer. Few individuals live more than 15 months (Moyle 1976).

Life history is flexible, varies with environmental conditions (Stearns 1983).