This study, conducted in the Dnieper-Bug Canal in Belarus, is the first to monitor the seasonal (June–November) dynamics of infection with the parasitic ciliate Ophryoglena sp. in a zebra mussel (Dreissena polymorpha) population. Mean population prevalence and intensity of infection varied, respectively, from 11 to 62\% and from 0.9 to 24.1 ciliates/mussel. Mean prevalence was highly correlated with mussel length in mussels \<20\ mm (R²=0.97) and was lower in larger mussels. Mean infection intensity in mussels 1–25 mm long was similarly correlated with their size (R=0.98), reached a maximum in the 20–25 mm size-class, and then sharply decreased, thus providing evidence, albeit limited, that high intensity of infection might be lethal. Transinfection of zebra mussels by Ophryoglena sp. was achieved in the laboratory—a first for a protozoan parasite of D. polymorpha; from an initial complete lack of infection, mean prevalence and intensity rose, respectively, to 86.7\% and 8.3 ciliates/mussel.