Shell growth, weight-specific growth of the soft tissue, and oxygen consumption were measured in native blue mussels, Mytilus spp., fromdifferent locations inDenmark, covering a salinity range from ∼10 to ∼30. The greatest growth rates were observed in mussels growing at average salinities of 25.7 and 29.5, the lowest rates occurred at the location exhibiting the most fluctuating salinity regime over time, with an average 20.5. Individuals in waters with a salinity of 25.7 also displayed the greatest condition index of all locations (12.1mg/cm³, P < 0.05).Mussels from five of six locations displayed similar oxygen consumption rates (P ≤ 0.83) when standardized to weight (range, 0.78–0.88 mg O₂/g/h. Of the salinities noted in the experiment, 25.7 appears to be the optimal salinity in terms of growth and condition, whereas strongly fluctuating salinity obviously involves reduced growth. At first glance, this study may appear to be just one among numerous attempts to describe the effect of salinity on growth and respiration in Mytilus edulis and its Baltic hybrids. However, themajority of studies focus on field transplants and responses to salinity alterations in the laboratory, whereas only sparse information exists on locally adapted blue mussels in relation to their ambient, native salinity.