The aim of this study is to elucidate the impact of age and temperature on metabolic key parameters (respiration rate and mitochondrial marker enzymes) controlling maximal size and longevity in the blue mussel Mytilus edulis L. from geographically separated populations of Northern European seas. Mussels from the Barents Sea attain similar maximal sizes as animals from the warmer North Sea. Young animals (<5 y) grow faster in the North Sea, however, inshore Barents Sea mussels grow rapidly even at advanced age. The slowest growth rates were found in a population exposed to strong wave action in the Barents Sea. Respiration rates followed the same age dependent pattern in mussels from all studied populations with low respiration rates in the youngest specimens followed by a rapid increase and then a gradual decline to nearly constant values in the oldest animals. When compared at a common temperature, metabolic rates only of young blue mussels (<7-8 y) are cold compensated with maximal respiration in the most Northern populations. Cold compensated metabolic activities in mussels from Northern populations may support similar patterns of maximal body size and longevity as in the more temperate Southern population.