Transstadial, transovarial and combined transstadial-transovarial transmission of Babesia microti in its tick vector, Ixodes ricinus, was studied. The ticks used (larvae, nymphs, females) were part of the progeny of nymphs collected in an area nearby Hanover. Specimens to be infected alimentarily were fed on parasitaemic Golden Hamsters (Mesocricetus auratus). The ability to infect mammalian hosts of a tick stage was examined by feeding it on negative Golden Hamsters. B. microti was found to be transmitted transstadially from the alimentarily infected larva to the following nymphal stage, only. Thus, this nymph is the only stage transmitting the Babesia to mammalian hosts. Transovarial transmission could not be observed.