ABSTRACTMating and infection structures ofUstilago maydisproduced in culture and on maize plants were examined with light and electron microscopy. Fusion tubes formed between low numbers of compatible haploid sporidia incubated in water for 8–16 hours. Both nuclei from paired sporidia migrated into a rapidly elongating hypha that did not differentiate further in the absence of host cells. Sporidial fusion and hyphal emergence also occurred rapidly and consistently in leaf whorls of inoculated maize seedlings, and hyphae had entered maize leaves after 12 hours. Only young leaves in the inner part of the leaf whorl were infected. Hyphae did not grow through stomatal openings, but, instead, entered epidermal cells by penetrating the cuticle and then either growing directly through the underlying cell wall or first growing between epidermal cells before penetrating an anticlinal wall of one of the cells. Invaded epidermal cells exhibited no necrosis or other dramatic response to entering hyphae.