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Eugregarines of the suborder Septatorina are apicomplexan parasites that are found mainly in arthropods. Some exceptions are species in the Metameridae that contains the only 5 septate gregarines recorded from annelids. The type genus is Metamera Duke, 1910 with two species, Metamera schubergi Duke, 1910, in European Glossiphonia complanata, and Metamera reynoldsi Jones, 1943, from North American G. complanata. Over the summers of 1995-1998, in Keith County, Nebraska, septate gregarines were found in the glossiphoniid leech Helobdella triserialis. The gregarines were determined to be a new species of Metamera, herein named Metamera sillasenorum. Measurements of size and body proportions of over 600 gregarines and 50 oocysts showed differences from measurements of M. schubergi and M. reynoldsi, and secondary septa in the deutomerite were rarely observed. Field observations indicated that M. sillasenorum is probably host specific. In the laboratory, leeches also exhibited strong feeding preferences; e.g., H. triserialis and G. complanata consumed only snails, whereas Helobdella stagnalis consumed only oligochaetes. Infection experiments demonstrated that freshwater snails ingest the oocysts and are required as mechanical vectors. Oocysts were passed unaltered through the snails' intestines. Glossiphonia complanata did not become infected regardless of heavy exposure to oocysts, although only 5 G. complanata were used in the experiments. The results show that host specificity of M. sillasenorum is most likely due to a combination of host-feeding habits and host-parasite compatibility.