Studies of climate change effects on individuals and populations are mostly couched in terms of lethal temperature limits, but 2 other elements make predictions difficult: the effects of sublethal stress and intraspecific variability. Sublethal stress, such as exposure to highly variable temperatures, can cause carryover effects that influence how an animal responds to subsequent events and its long-term fitness. Additionally, intra-specific genetic variability can allow more complex species responses to climate change. We used 2 genetically distinct lineages of the intertidal mussel Perna perna to investigate the effects of sublethal temperature variability and genetic identity on thermal sensitivity estimates derived from standard metabolic rate measurements. Mussels were exposed to one of 3 acclimation treatments: measurements done immediately after collection (i.e. field fresh), or exposure to 2 wk of either constant temperature or fluctuating temperatures. The mussels were then exposed to rising temperatures within the range normally experienced in the field while oxygen consumption was recorded. We found no effect of lineage or treatment on the aerobic standard metabolic rate of mussels, but Q10, a proxy for thermal sensitivity, was influenced by treatment. While Q10 values under constant temperature did not differ significantly from those for field-fresh mussels, both were significantly lower than for fluctuating temperatures. Thus, temperature variability increased thermal sensitivity, but our results indicate that this was independent of genetic identity. Chronic increased temperature variability and repeated exposure to high (but sublethal) temperatures under climate change are likely to increase mussel temperature sensitivity and decrease performance. Such alterations to fitness are expected to occur before animals are exposed to lethal temperatures.