Classically, infective development of Trypanosoma (Trypanozoon) brucei in tsetse flies is thought to take the route crop-midgut-hindgut proventriculus-hypopharynx-salivary gland, where the parasites reach their infective phase. It has been shown experimentally that T. (T.) brucei is capable of developing up to the infective stage in G. morsitans following inoculation of bloodstream form trypanosomes into the haemocoel. The rabbit on which flies were maintained became infected 18 days after exposure to the bite of experimentally inoculated flies. The possibility that T. (T.) brucei may be transmitted cyclically from tsetse flies to a mammalian host without necessarily following the classical, prescribed route is discussed. Apart from the normal longitudinal binary fission, various modes of multiplication were observed among trypanosomes in the haemocoel, modes which have not been observed previously in the tsetse fly.