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An extensive community of organisms inhabits the common periwinkle, Littorina littorea (L., 1758), along the waveswept rocky shores of Canada's East coast. This community, which includes both facultative and obligate endosymbionts, comprises a diverse array of species from seven animal phyla, including Annelida, Arthropoda, Gnathostomulida, Nematoda, Nemertea, and Platyhelminthes, as well as ciliates and algae. The presence of larger numbers of endosymbionts was found to correlate with specific shell characteristics of the snail host, including a wider aperture and columella, suggesting that these individuals have a larger mantle cavity relative to snails housing a small community of endosymbionts. Snails with large communities of endosymbionts were usually encrusted with coralline algae and often had trematode infections. Although L. littorea has been extensively studied since the last century, the existence of this community of organisms has passed unnoticed. The large diversity of organisms in this community suggests that these snails may provide refugia for a wide range of smaller taxa. Key words: endosymbiosis, parasitism, commensalism, Mollusca, Littorinidae. Resume : Une vaste communaute d'organismes vit dans le bigorneau Littorina littorea (L., 1758) le long des cotes balayees par les vagues de la cote est du Canada. Cette communaute, qui compte des endosymbiotes facultatifs et obligatoires, comprend un eventail varie d'especes appartenant a sept phylums animaux, dont des annelides, des arthropodes, des gnathostomulides, des nematodes, des nemertes et des plathelminthes, en plus de cilies et d'algues. La presence d'un grand nombre d'endosymbiotes est correlee a certaines caracteristiques de la coquille des mollusques hotes, dont une ouverture et une columelle relativement larges, indiquant possiblement de plus grandes cavites du manteau que celles d'individus abritant de plus petites communautes d'endosymbiotes. Les bigorneaux contenant d'importantes communautes d'endosymbiotes etaient generalement incrustes d'algues corallines et presentaient des infections a trematodes. Bien que L. littorea ait fait l'objet de nombreuses etudes depuis le siecle dernier, l'existence de cette communaute d'organismes n'avait pas ete decelee auparavant. La grande diversite des organismes qui la constituent suggere que ces bigorneaux pourraient servir de refuge a un vaste eventail de petits taxons. [Traduit par la Redaction] Mots-cles: endosymbiose, parasitisme, commensalisme, mollusques, littorinides., Introduction The term symbiosis, first coined by De Bary in 1879 (cited in Vega et al. 2006) to include parasitism, as well as commensalism and mutualism, also includes epibiosis and [...]