The development of the piroplasm Babesia equi was studied by light microscopy in the salivary glands of three different Hyalomma species during and after the engorgement of nymphs on experimentally infected horses and after adults had fed on a vertebrate host following ecdysis. The stock of B. equi used was isolated from a horse imported from Turkmenistan (CIS) in 1991. The findings, being identical in all three Hyalomma species, differ with regard to the chronological order of the development stages in several respects from the results of previous studies based upon light or electron microscopy. A first sporogony phase of B. equi was found to develop in the salivary glands of the engorged nymphs before the ticks moulted to adults. Beginning at day 6 postinfestation (p. infest.) of the nymphs, spindle-shaped sporozoites appeared to be formed by both rapid sequential fission of a multinucleated complex and a process of radial budding from multiple fission bodies. Sporozoites isolated from the salivary glands of the engorged nymphs proved to be infectious when they were injected into a susceptible horse. After the nymphs had moulted, a second sporogony phase similar to the first one observed in the salivary glands of engorged nymphs could also be initiated in the salivary glands of adults when they were attached to a vertebrate host. Sporozoites produced in the salivary glands of adults were equally infectious for horses. Thus, two completely separate sporogony phases in B. equi seem to develop successively in the salivary glands of Hyalomma species during a transstadial transmission.