The blue mussel Mytilus edulis is described as second intermediate host for Echinostephilla patellae from the common limpet Patella vulgata. Mussels were infected with metacercariae of E. patellae under laboratory conditions. Average infection rates increased with increasing temperature, whereas numbers of cercariae, to which individual mussels were exposed, had no effect on relative infection success. The round to slightly oval metacercariae with an average cyst diameter of 208 mu m (range 186-243 mu m) encysted exclusively in the foot tissue of M. edulis. Morphologically similar metacercariae were found in naturally infected mussels at sites where parasitized P. vulgata and M. edulis are sympatric. This is the first report of E. patellae in blue mussels. The detection of M. edulis being a second intermediate host is of particular interest with regard to the abundance of the parasite and host organisms in intertidal rocky shore ecosystems. The potential role of the common limpet P. vulgata as an alternative secondary host is discussed.