Biochemical mutants of Ustilago hordei (Pers.) Lagerh were used to infect barley seeds in a study of the pathway of infection and the development of the parasite within the host. Microscopic studies in which the parasite was recovered from pieces of host tissue, identified and grown in vitro confirmed earlier views that the parasite mycelium was present throughout the spike primordia of the barley host.From genetically marked infections, it became evident that diploidization and subsequent recombination and haploidization may follow the dikaryotic stage of the parasite.