This study quantifies the infection prevalence and intensity of the European, commensal, host-specific ciliateConchophthirus acuminatus(Scuticociliatida: Conchophthiridae) in five zebra mussel populations within the Republic of Belarus. Laboratory and field experiments were also conducted to assess variables affecting infection.C. acuminatuswas found in zebra mussels in all five waterbodies sampled: Naroch, Myastro, and Lukomskoe Lakes, Skema Stream, and in the Svisloch River. Prevalence was always 100\%, with the exception of shallow areas (<=0.5 m depth) in the Svisloch River. This was possibly the result of the elimination of the majority of infected zebra mussels each winter by a combination of factors, including ice scour, mallard duck predation, and fluctuating water levels. As a result, zebra mussels are not permanently present at shallow depths, and the mussels that we randomly sampled there during the summer were thus smaller (i.e., younger) and less infected than those present in deeper sections of the river. This is the first study to quantify the intensity ofConchophthirusinfection in zebra mussels. When infection prevalence was 100\%, it was not uncommon for zebra mussels to have 500–2,000C. acuminatusin their mantle cavities, particularly those mussels ca. 15–30 mm in length. Zebra mussels, while relatively small bivalves, have one of the highest intensities ever reported for ciliates in the order Scuticociliatida. Infection intensity correlated directly with mussel length (range inr²\ =\ 0.83–0.92). Transinfection of zebra mussels withC. acuminatuswas achieved both in the laboratory and field, and represented the first successful trials to initiate protozoan infection inDreissena.Laboratory experiments demonstrated thatC. acuminatusrapidly leave their dying zebra mussel hosts, and this suggested that these mussels are likely a major source for the spread ofC. acuminatusinfection. Field trials indicated that the presence of mussels with high intensity infections can cause an increase in the levels of infection (both prevalence and intensity) in other zebra mussels in their microhabitat.