Mytilicola intestinalis, a copepod parasite of the gut of mussels, is endemic along European coasts. It has been responsible for heavy mortalities, especially in Mont Saint-Michel's Bay (Northern Brittany) between 1982 and 1984. From laboratory screening of seven drugs, an organophosphorate Dichlorvos appears to be the most efficient. Dichlorvos treatment for 2 h at 30 mg/l concentration frees mussels from Mytilicola without any mortality. Later field trials showed an equal efficiency if the drug is used properly. © 1992.