SUMMARYThe sporulation ofExserohilum(=Helminthosporium)rostratumoccurred in the dark at temperatures from 28 to 34 C. Light induced sporulation at lower temperatures, enhanced sporulation at 28 and 31 C, but inhibited the terminal phase of sporulation at 34 C. In dark-grown cultures, spores were characteristically elliptical and spore length increased with temperature to 31 C. Under continuous light, characteristic rostrate spores were formed at all temperatures tested, spore size increasing with temperature to 24 C, and then decreasing slightly with further temperature increases to 31 C. At 24 C, length of spores formed under continuous fluorescent irradiation was more than three times those formed in the dark. Under alternating light and dark conditions, both elliptical and rostrate conidia were formed as well as an intermediate, cylindrical spore type. Sporulation and spore length increased with light intensity. Added glucose (up to 4%) in vegetable-juice agar tended to shorten spores in light-grown culture.