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Two microsporidia, Nosema slovaca Weiser et Rehacek, 1975 and Unikaryon ixodis (Weiser, 1957) comb. n. infect Ixodidae in Central Europe. Their distribution, pathology and ultrastructure were studied in material assembled during an extensive survey of rickettsial infections in ticks collected in Slovakia, the Czech Republic and Hungary. Ixodes ricinus and Dermacentor reticulatus were the natural hosts of N. slovaca while I. ricinus is the only host for U. ixodis in the field. Both microsporidia are infectious by injection also for other hard ticks occurring in Central Europe. Transmission occurs in nature via infected eggs and by ingestion of spores. N. slovaca has been confirmed as belonging to the genus Nosema. It produces spores of two types, with 18 and 9 turns of the polar filament, respectively. On the basis of spore ultrastructure the other microsporidian with smaller spores, described as Nosema in 1957, is transferred to the genus Unikaryon Canning, Lai et Lie, 1974. It also produces two types of spores, with 6 and 4 turns of the polar filament, respectively.