In the wheat plants infected with dwarf bunt, sporulation occurs only in developing kernels. The dikaryon was isolated from infected kernels and from the rachis of infected spikes. The development of pathogenic hyphae and teliosporogenesis in vivo and in vitro is described. Ultrastructural studies indicated that in vivo teliospore primordia detach from sporogenous hyphae in the hymenium layer and develop into mature teliospores. Teliospores were borne terminally on sporogenous hyphae in vitro. Substances that stimulate hyphal branching and inhibit teliospore formation of the dikaryon were isolated from vegetative wheat tissues but not from bunt-infected spikes containing the sporulating dikaryon. The dikaryon resumed sporulation only when transferred to rich nutrient media without extracts. Extracts from infected wheat spikes containing the sporulating dikaryon induced the vegetative dikaryon to sporulate. The bioregulators involved in these phenomena were not identified. Bioactive extracts had no apparent effect on the growth of the monokaryon. A fluorochrome, mithramycin, was used to detect nuclei in monokaryotic and dikaryotic hyphae. New approaches to bunt control are discussed. Key words: Tilletia caries, Tilletia controversa, monokaryon, teliospores, bioregulators.