The musselMytilus edulis has been widely used as a bioindicator of marine pollution. Because of its wide distribution around the world, it has been possible to establish a global mussel watch to monitor pollutants in the marine environment. Recent taxonomic discoveries based on studies of allozyme variation, however, have shown thatMytilus edulis is actually a complex consisting of three separate evolutionary lineages which deserve the ranks of separate species: 1) the AtlanticMytilus edulis 2)Mytilus galloprovincialis and 3)Mytilus trossulus. Many mussels previously classified asMytilus edulis can be reclassified as eitherMytilus galloprovincialis orMytilus trossulus. In the present study, specimens ofMytilus edulis andMytilus trossulus were collected from the same habitat and analyzed for the concentrations of 25 elements by inductively coupled plasma mass spectrometry (ICP-MS).Mytilus trossulus had higher element concentrations thanMytilus edulis. Differences between the element concentrations of the two species were related to differences in growth rate rather than to any direct differences between the element metabolisms of the species. It is suggested that such differences could be corrected by standardizing the mussels to a fixed width/height ratio. In order to avoid errors in future monitoring programs, it is recommended that taxonomic variables should be considered.