The association between Mytilus chilensis and the isopod Edotea magellanica in two natural banks of Mytilus chilensis from the intertidal area of the Straits of Magellan (southern Chile) is described. This may be the first documentation of such an association between a bivalve mollusc and an isopod. In Gregorio and Santiago Bays, 18.3% and 7.6%, respectively, of the mussels investigated were occupied by isopods. The highest degree of isopod occupation was found in mussels between 30 mm and 45 mm in shell length. The regression lines for the relationship between shell length and dry-tissue weight of mussels with and without isopods showed that the isopods have no influence on the meat content of the mussels. Edotea magellanica is a non-obligatory commensal, nourished on food that is associated with the host's activity (formation of pseudofeces) but that is not part of the host's food. The life cycle of Edotea magellanica was schematically constructed using the available data. The complete life cycle of the...