Physiological traits are key determinants of species success in any given environment. Competing metabolic demands can influence success by limiting the available scope for aerobically-demanding activities, including reproduction and dispersal. The present study investigated whether the aerobic performance of Gambusia holbrooki, one of the most invasive species in the world, was affected by the embryonic demands of pregnancy and whether these demands were compounded in a hypoxic environment. To quantify these effects, we measured the resting metabolic rate (RMR) and critical swimming speed (U ) of wild-caught adult mosquitofish in saturated oxygen conditions, and following chronic exposure to hypoxic or hyperoxic water. Contrary to expectations, the RMR and U of gravid females was equal to that of non-gravid females and males, once the significant effects of body mass on these traits was accounted for. Whilst the exposure studies demonstrated that RMR changes subject to oxygen availability, overall aerobic performance does not. These findings suggest that mosquitofish metabolism is adapted for continued reproductive function and dispersal in low oxygen environments, which are essential for establishing and replenishment of populations. The ability of this species to tolerate hypoxia may confer a competitive advantage relative to native species, especially in disturbed lentic environments where prolonged periods of hypoxia are more common. Given their live-bearing mode of reproduction and resilience to the metabolic demands of reproduction, even in low oxygen conditions, we suggest that the physiology of female mosquitofish is a significant contributor to the invasion success and persistence of the species. crit crit