SUMMARYConidium development ofExserohilum rostratumwas partially inhibited by light at 28 and 31 C, and completely inhibited at 34 C. Older conidiophores were very sensitive to inhibitory wavelength and readily dedifferentiated into vegetative aerial mycelia. Light was effective in stimulating spore elongation during the formation of conidiophores as well as during the development of conidia. Exposure to light during the formation of conidiophores in the inductive phase resulted in the formation of rostrate conidia in the subsequent dark-terminal phase. The effect was greatest in young conidiophores incubated below 24 C. The phenomenon was not evident when older conidiophores were exposed to light.