Fish species from temperate climates exhibit seasonal variation in reproduction triggered by abiotic variables such as day length and temperature. The sailfin molly Poecilia latipinna occupies habitats in the southern temperate zone in North America, but little is known about seasonal patterns in reproductive parameters such as female egg production and male sperm production, especially in spring-fed habitats. Males and females were collected monthly for 21 mo to: (1) examine the seasonal patterns in egg availability, (2) examine the relationship between female size and fecundity, (3) examine seasonal patterns in male sperm availability and (4) examine the relationship between male size and sperm availability. We found that reproductive investment varied seasonally in both males and females in a population of sailfin mollies occupying a spring-fed habitat. Because both egg and sperm production can be energetically costly, individuals may reduce costs associated with reproduction by reducing the availability of ready gametes during the time of year when mating is infrequent.