Fluctuating temperature patterns due to climate change could negatively impact the survival of species whose sex is determined by environmental cues rather than genetic factors. Species whose sex is determined by ecological cues or social interactions fall under the Environmental Sex Determination (ESD) category, while the Genetic Sex Determination (GSD) category includes species whose sex is determined by genotype at conception. There are numerous published examples of teleost fish species that rely on ecological factors for sex determination, but no studies have explored the extent to which the annual killifish Austrofundulus limnaeus, a species without sex chromosomes, relies on ESD. A. limnaeus inhabit ephemeral ponds in Venezuela, South America that experience extreme daily fluctuating temperatures. Climate change could affect this delicate lifecycle by disrupting seasonal temperature patterns that alter sex ratios or exposure to temperatures beyond their tolerance range. Therefore, it is essential to determine what impact temperature and social interaction may have on sex determination in developing A. limnaeus. To explore ESD, A. limnaeus larvae were exposed to constant temperatures of 20 and 30°C, and a daily fluctuating temperature regime from 20-30°C from hatching until the display of secondary sexual characteristics. Some fish were grown in isolation while others were grown in small groups. Fish grown in isolation at 30°C exhibited female-biased sex ratios with 64% developing as females whereas 69% in 20°C and 61% in fluctuating temperatures exhibited male-biased sex ratios. Ongoing experiments are being conducted to explore the possible interaction between temperature and social cues for determining the sex in this species. A. limnaeus has a relatively higher tolerance to fluctuating temperatures due to their ephemeral pond environment, and thus represents an opportunity to explore how even the most tolerant of species may respond to global climate change, especially in the context of ESD.