Atlantic cod are a very temperature sensitive species of fish. Previous phylogenetic and phenotypic studies have established different populations of Atlantic cod that have different metabolic rates temperature preferenda. Two cod populations, North Sea cod and North Eastern Arctic cod, were acclimated to two temperatures, 4 and 10°C. Part one of this study examined the influence of temperature on myoglobin expression in cardiac tissue. Cardiac myoglobin mRNA levels demonstrated a significant negative response to increasing temperature. This trend carried over to myoglobin protein expression levels though differences were not significant. Part two examined the influence of temperature on myosin heavy chain in both cardiac and skeletal muscle. Both the amount of extractable cardiac MHC and its activity were unaffected by temperature acclimation, with all groups showing similar activities. However the amount of extractable skeletal muscle MHC showed a highly significant five fold increase in 4°C acclimated cod. Although the activity when measured on a per mg protein basis is significantly lower, on a per tissue basis however there was a significant increase in the MHC activity in 4°C acclimated cod. Although there were no differences found between the two populations, both cardiac myoglobin and skeletal, though not cardiac MHC proved to be responsive to temperature in Atlantic cod.