he dynamics determining the establishment and maintenance of epibiotic relationships are fundamental to understand the role of biological interactions in the functioning of marine coastal ecosystems. Epibiosis of barnacles on mussels is a common feature of many intertidal rocky shores. There are several possible explanations for this relationship, but it is unclear whether the association is commensal, antagonistic, or mutualistic. We investigated the diets of barnacles and mussels in an epibiotic relationship (epibiont barnacle and basibiont mussel), and compared them to the diets of conspecifics living on the same rocky shore but independently (free barnacle and free mussel), using stable isotope (SI) and fatty acid (FA) techniques. The SI results indicated that the diet of mussels did not change as a function of epibiosis, while free barnacles were characterized by higher δ15N values than epibiotic conspecifics, indicating that they were feeding on more recycled food sources. This result was, however, supported by data from only one site. Results from FA were partially consistent with those of SI analysis, indicating that mussel FA profiles did not change between basibiont and free mussels, but that epibiotic barnacles were characterized by a lower amount of polyunsaturated FA than free barnacles. This suggests that the diet of epibiotic barnacles is of lower quality. This could be due to competition for food with mussels, driven by the different mechanisms of food assimilation between epibiotic barnacle and basibiont mussel; or by the different sizes of particles ingested by the two species under epibiosis vs free-living conditions. The discrepancy between the FA and SI results observed could reflect the different integration time of the analytical techniques, with FA reflecting shorter term variability than SI. We conclude that epibiosis represents an unfavourable relationship for barnacles due to the lower quality of food available when colonizing mussel shells, whereas mussels neither benefit nor were harmed by this association. Thus, this relationship is amensalistic: negative for barnacles but neutral for mussels.