Composition of the fauna of organisms associated with Mytilus edulis in the Kandalaksha and Onega Gulfs of the White Sea has been examined. The following 8 symbiotic species were revealed: Choricystis sp. (Chloro[hyceae), Peniculistoma mytili, Ancistrum mytili (Ciliata, Oligohymenophorea), Urastoma cyprinae, Paravortex sp. (Platyhelminthes, Rhabditophora), and metacercaria of Cercaria parvicaudata, Himasthla sp., and Gymnophallus sp. (Platyhelminthes, Trematoda). Besides, different free-living organisms were found in the mantle cavity; 6 species of nematodes, the planktonic copepod Microsetella norvegica, undetermined copepods, isopod Jaera sp., sea mites of the family Halacaridae, and chironomid larvae. Parameters of infestation and places of localization in the host are given for each group of organisms. Some regularities in the horizontal and vertical distribution of organisms associated with M. edulis are noted.