The strontium, magnesium, zinc, and copper contents of the aragonitic shells of Macoma balthica L. (Pelecypoda), from the west and Baltic coasts of Sweden, were determined by atomic absorption spectrophotometry. The shells from the less saline environment (Baltic) yielded higher average values for Sr. The shells from the more saline environment gave higher values for Mg and Cu. The Zn content did not differ significantly in the two environments.