Metabolic thermal plasticity is central to the survival of fishes in a changing environment. The eurythermal three-spined stickleback Gasterosteus aculeatus displays thermal plasticity at the cellular level with an increase in the activity of key metabolic enzymes in response to cold acclimation. Nonetheless, it is unknown if these changes are sufficient to completely compensate for the depressive effects of cold temperature on whole organismal metabolic rate (ṀO2). The authors hypothesized that as a cold-tolerant, eurythermal fish, absolute aerobic scope (AAS), the difference between the maximum metabolic rate (MMR) and standard metabolic rate (SMR), would be maintained in G. aculeatus following acclimation to a range of temperatures that span its habitat temperatures. To test this hypothesis, G. aculeatus were acclimated to 5, 12 and 20(C for 20–32 weeks, and SMR, MMR and aerobic scope (AS) were quantified at each acclimation temperature. The maximal activity of citrate synthase (CS), a marker enzyme of aerobic metabolism, was also quantified in heart ventricles to determine if cardiac aerobic capacity is associated with AS at these temperatures. SMR increased with acclimation temperature and was significantly different among all three temperature groups. MMR was similar between animals at 5 and 12(C and between animals at 12 and 20(C but was 2.6-fold lower in fish at 5(C compared with those at 20(C, resulting in a lower AAS in fish at 5(C compared with those at 12 and 20(C. Correlated with a higher AAS in animals acclimated to 12 and 20(C was a larger relative ventricular mass and higher CS activity per 100 g body mass compared with animals at 5(C. Together, the results indicate that despite their eurythermal nature, AS is not maintained at low temperature but is associated with cardiac aerobic metabolic capacity. [ABSTRACT FROM AUTHOR]