SUMMARYMarked differences in the characters ordinarily used in taxonomy are revealed when 10 strains of 3 species (F. Solani, F. oxysporumandF. avenaceum), representing 3 sections of the genusFusarium, are grown in light, and in darkness.The quality of the colony and morphologic characters were affected, as were also the quantity and to a certain degree the occurrence of such characters as perithecial primordia.Evidence is given which indicates that the effect of light is produced on that portion of the thallus which is actively growing at the time of exposure. Single spore cultures subjected to light only for the first 4 days of growth, fail to develop in the same manner as those allowed to remain in light.It is concluded that such characters as color, zonation, type of colony, presence or absence of sporodochia; size, shape and septation of macroconidia; and even the occurrence of a perithecial stage, can not be employed successfully in taxonomy unless these fungi are grown in adequate light.