Climate change has led to increases in global mean temperatures, as well as in the intensity, frequency, and duration of extreme temperature events. Notably, the unpredictability of thermal fluctuations is increasing in natural ecosystems. Thus, experiments that incorporate realistic, unpredictable thermal variability should lead to more accurate predictions of how temperature will impact physiological performance in natural populations. Using the hermaphroditic, amphibious Mangrove rivulus (Kryptolebias marmoratus) as a model species, I investigated the difference between warming predictable and unpredictable thermal fluctuations on physiology and also tested for genetically based differences in phenotypic plasticity among lineages. I hypothesized that compared to predictable thermal variability, unpredictable thermal variability would negatively affect acclimation capacity. To test this, I acclimated three isogenic lineages of Mangrove rivulus in three thermal groups: predictable (27°C-35°C) and unpredictable thermal fluctuations (27°C -35°C) with a similar thermal load, and stable control temperature (27°C). After five weeks of acclimation, I measured growth rate, thermal biology endpoints (emersion temperature, critical thermal maximum (CTmax)), and gill morphology (interlamellar cell mass height, lamellar width/height). I determined that fish acclimated to either thermal fluctuation treatment (27°C-35°C) had a lower growth rate than control fish at a stable temperature, yet thermal tolerance was not impacted by either warming temperature cycle. Notably, fish acclimated to either predictable or unpredictable thermal cycle had a reduced gill surface area when compared to control fish, possibly as a preparatory response for emersion. The results of my thesis led me to develop a collaborative second project that compared two environmentally relevant unpredictable thermal cycles to the mean of the cycle, with the goal of disentangling the effects of thermal fluctuations and thermal warming. We acclimated three isogenic lineages of Mangrove rivulus to four different thermal regimes – two unpredictable thermal cycles, one mimicking current climate conditions (low cycle: 24°C-29°C) and one predicting warming conditions (high cycle: 29°C -34°C), and the mean of both cycles (low mean: 26.5°C, and high mean: 31.5°C). We measured growth, fecundity, and thermal tolerance and predicted that (1) these variables would be similar in fish acclimated to unpredictable thermal fluctuations when compared to fish acclimated to the mean of the cycle, and (2) these variables would be different between the two unpredictable cycles, with fish in the warming fluctuations having decreased reproductive output, growth, and thermal safety margins as they were more frequently exposed to suboptimal temperatures. Our preliminary results supported both predictions, as cyclic acclimated fish responded similarly to fish exposed to the mean of the cycle, and fish in both warming conditions (high cycle and high mean) had decreased growth and reproductive output compared to fish in the cooler thermal groups. Similar to the results of my thesis, thermal tolerance was conserved across groups. Collectively, my data reveal the importance of considering daily, unpredictable thermal variability when studying the effects of temperature on fishes. Additionally, genetically based differences were observed in response to thermal unpredictability, suggesting that an organism’s response to changing environmental conditions is genotype specific.