Directional changes in temperature have well-documented effects on ectotherms, yet few studies have explored how increased thermal variability (a concomitant of climate change) might affect individual fitness. Using a common-garden experimental protocol, we investigated how bidirectional temperature change can affect survival and growth of brook trout (Salvelinus fontinalis) and whether the survival and growth responses differ between two populations, using four thermal-variability treatments (mean: 10?°C; range: 7-13?°C): (i) constancy; (ii) cyclical fluctuations every two days; (iii) low stochasticity (random changes every 2 days); (iv) high stochasticity (random changes daily). Recently hatched individuals were monitored under thermal variability (6 weeks) and a subsequent one-month period of thermal constancy. We found that variability can positively influence survival, relative to thermal constancy, but negatively affect growth. The observations reported here can be interpreted within the context of Jensen's Inequality (performance at average conditions is unequal to average performance across a range of conditions). Projections of future population viability in the context of climate change would be strengthened by increased experimental attention to the fitness consequences of stochastic and non-stochastic thermal variability.