Dreissena polymorpha were dissected and examined for endosymbionts from 17 waterbodies in Belarus — the country through whose waterways zebra mussels invaded Western Europe nearly two centuries ago. Fourteen types of parasites and other symbionts were observed within the mantle cavity and/or associated with internal tissues, including ciliates (Conchophthirus acuminatus, Ancistrumina limnica, and Ophryoglena sp.), trematodes (Echinostomatidae, Phyllodistomum, Bucephalus polymorphus, and Aspidogaster), nematodes, oligochaetes, mites, chironomids, and leeches. Species composition of endosymbionts differed among river basins and lake systems. The most common endosymbiont was the ciliate C. acuminatus. Its mean infection intensity varied significantly among waterbodies from 67 \textpm 6 to 3,324 \textpm 556 ciliates/mussel.