Ophryoglena hemophaga n. sp. is described from a freshwater Dreissena polymorpha population in the Rhine delta of the Netherlands. This is the first ophryoglenine species (order Hymenostomatida, suborder Ophryoglenina) recorded as a molluscan parasite. As is typical of ciliates in the suborder Ophryoglenina, O. hemophaga exhibits a polymorphic life history with cystment and reproduction by palintomy. Trophonts were observed within digestive gland lumina, and zebra mussel hemocytes were present in some of their digestive vacuoles. The presence of a single, longitudinal tract of multiple contractile vacuoles represents its most unique feature and distinguishes it from all other described Ophryoglena spp. The number of somatic kineties of O. hemophaga (range 50 to 62) is also a distinguishing feature, since it is the lowest described from any Ophryoglena sp. Other characteristics of this species include: ovoid to elongate trophonts 96 to 288 \textmum in length, with an elongate macronucleus 41 to 65 \textmum in length; tomonts 50 to 150 \textmum in diameter producing a clear mucous cyst envelope, whose thickness is approximately half of the tomont diameter; elongated theronts 96 to 131 \textmum in length which emerge after 1 to 3 cell divisions taking 36 to 48 h at 20 \textpm 3°C. Protomonts and theronts are, respectively, negatively and positively phototactic—characteristics that likely aid in maintenance of infection in zebra mussel populations.