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Among the potential biotic factors affecting population dynamics, parasitism has received relatively little attention. The purpose of this study was to inventory marine bivalve parasites and to investigate relations between different parasite communities. Five intertidal stations along the French Atlantic shore were studied between Brittany and Arcachon Bay. Two bivalves (the edible cockle, Cerastoderma edule, and the Manila clam, Ruditapes philippinarum) were sampled and examined. The parasites included a bacterium (Vibrio tapetis), a protozoan (Perkinsus sp.), and digenetic trematode metacercariae (10 species). Perkinsus sp. prevalence and density in clams were higher than those measured in cockles (30?250 vs. 36?cells?g-1 of gill wet weight, respectively) while digeneans were more common in cockles (46 metacercariae per cockle host vs. 1 per clam host). Distributions of digeneans and Perkinsus sp. were similar along the Atlantic shore. However, at each sampling station and at the individual scale, no correlation between Perkinsus sp. and digenean abundance was detected. These results suggest different ecological niches used by these two parasites within a community of bivalves but a similar response to environmental factors such as temperature and salinity. Vibrio tapetis was found at relatively high prevalence in all stations for both bivalve species (from 17% to 43% in cockles and from 23% to 50% in clams), but was associated with a low prevalence of Brown Ring Disease (BRD), a shell disease caused by this pathogen. No statistically significant relationship of these parasitic bacteria with trematodes and Perkinsus sp. was evidenced in the present study. Parmi les facteurs biotiques pouvant affecter la dynamique de populations, le parasitisme a re?u relativement peu d''attention. Le but de cette ?tude est d''inventorier les parasites de bivalves marins et de rechercher les relations entre les diff?rentes communaut?s de parasites. Cinq stations intertidales le long du littoral fran?ais, entre la Bretagne et la baie d''Arcachon, ont ?t? ?tudi?es. Deux esp?ces de bivalves, la coque (Cerastoderma edule) et la palourde (Ruditapes philippinarum) ont ?t? ?chantillonn?es et examin?es. Les parasites pr?sents sont une bact?rie (Vibrio tapetis), un protozoaire (Perkinsus sp.) et des m?tacercaires de tr?matodes dig?nes (10 esp?ces). La pr?valence et la densit? de Perkinsus sp. dans les palourdes sont sup?rieures ? celles mesur?es pour les coques (30 250 vs. 36 cellules g-1 de branchies, respectivement) alors que les tr?matodes dig?nes sont plus communs dans les coques (46 m?tacercaires par coque vs. 1 pour les palourdes). La r?partition g?ographique de Perkinsus sp. et celle des tr?matodes dig?nes sont similaires le long du littoral atlantique. Cependant, lorsque l''on consid?re s?par?ment chaque station d''?chantillonnage, aucune corr?lation entre Perkinsus sp. et les tr?matodes dig?nes en terme d''abondance n''a ?t? d?tect?e ? l''?chelle individuelle. Ces r?sultats sugg?rent l''utilisation de niches ?cologiques diff?rentes par Perkinsus sp. et les tr?matodes dig?nes mais une m?me r?ponse aux caract?ristiques environnementales comme la temp?rature et la salinit?. Des pr?valences relativement importantes de Vibrio tapetis ont ?t? trouv?es ? chaque station et pour les deux esp?ces de bivalves (de 17 % ? 43 % dans les coques et de 23 % ? 50 % dans les palourdes), n?anmoins, elles sont associ?es ? de faibles pr?valences des sympt?mes de la maladie de l''anneau brun (MAB), une maladie de la coquille caus?e par ce pathog?ne. Aucune relation statistiquement significative n''a pu ?tre ?tablie entre la pr?sence de cette bact?rie et celle des deux autres parasites (tr?matodes, Perkinsus sp.). [ABSTRACT FROM AUTHOR]