A total of 6,199 adult ticks (Haemaphysalis intermis, Dermacentor reticulatus, D. marginatus and Ixodes ricinus) collected in three localities in Slovakia were examined for microsporidian infection. The spores of Nosema slovaca were found in six D. reticulatus (0.097%). The microsporidian experimentally inoculated into the hemocele of half-engorged females of D. reticulatus caused acute infection and death of the host. The infection can be transmitted also to other tick species from the same region. The yield of spores from one tick ranges from 3 X 10(5) to 52 X 10(7) depending on the infection dose and tick species. Peroral infection of some butterfly larvae with the microsporidian pores was unsuccessful. The importance of Nosema infection in ticks in nature is discussed.