Like many organisms, individuals of the freshwater ostracod species Eucypris virens can have either obligate sexual or asexual reproductive modes. Both types of individual routinely co-occur, including in the same temporary freshwater pond (their natural habitat in which they undergo seasonal diapause). Given the well-known two-fold cost of sex, this begs the question of how sexually reproducing individuals are able to co-exist with their asexual counterparts in spite of such overwhelming costs. Environmental stochasticity in the form of “false dawn” inundations (where the first hydration is ephemeral and causes loss of early hatching individuals) may provide an advantage to the sexual subpopulation, which exhibits greater variation in hatching times following inundation. We explore the potential role of environmental stochasticity in this system using life-history data analysis, climate data and matrix projection models. In the absence of environmental stochasticity, the population growth rate is significantly lower in sexual subpopulations. Climate data reveal that “false dawn” inundations are common. Using matrix projection modeling with and without environmental stochasticity, we demonstrate that this phenomenon can restore appreciable balance to the system, in terms of growth rates. This provides support for the role of environmental stochasticity in helping to explain the maintenance of sex and the occurrence of geographic parthenogenesis.