Although considerable research has been carried out on Dreissena spp., relatively little is known about the symbiotic organisms living within them. We report a two-year study of the dynamics of infection by the commensal ciliate Conchophthirus acuminatus in D. polymorpha and D. bugensis populations in the Dnieper River in the Ukraine and in four D. polymorpha populations in Belarus. C. acuminatus was present in all samples. The moderate to high infection prevalence and moderate infection intensity that we recorded in D. polymorpha populations were in contrast to the low prevalence and very low intensity of infection observed in D. bugensis. Recording C. acuminatus in D. bugensis represents the first record of any ciliate species from this mussel and the first report of C. acuminatus in a host other than D. polymorpha. Since infection prevalence and intensity strongly correlated with the size of D. polymorpha, the presence of large, infected mussels is likely important to serve as a reservoir for maintaining infection in the overall population. To infect new host mussels, C. acuminatus must disperse into surrounding waters, and we suggest that a period of mass dispersal may be synchronized to occur when new potential hosts, i.e., juvenile mussels, become abundant.