The development of the piroplasm Babesia equi was studied by light microscopy in the gut and the haemolymph of three different Hyalomma species during and after the nymphs had engorged on parasitaemic horses. The stock of B. equi used was isolated from a horse imported from Turkmenistan (CIS) in 1991. The existence of gamogony was identified by the occurrence of gamonts and gametes in the gut contents of the nymphs at between 3 and 4 days after infestation of the nymphs, before the ticks dropped off the experimentally infected horses. Zygotes and kinetes were observed in the intestinal cells from day 4 until day 7 after infestation i.e. 2 days prior to engorgement until repletion of the nymphs. Simultaneously, kinetes could also be seen in the haemolymph of engorged nymphs. The morphology and the sequence of the developmental stages of B. equi identified in the gut and the haemolymph of the vector ticks were identical in all three Hyalomma species. They resembled typical developmental stages of bovine Theileria species. Because of the close similarity of the developmental cycle of B. equi in Hyalomma species to the life cycle of Theileria species and due to the differences to the other Babesia species, a discussion about classifying B. equi into the family of Theileriidae now seems to be justified.