Colletotrichum graminicola produced two types of conidia in culture and during infection of corn leaves. One was lunate to falcate and was produced blastically from morphologically distinct conidiogenous cells. The second type was oval to elliptic, variable in size but smaller than falcate conidia, and was produced blastically from hyphae that lacked distinct conidiogenous cells. Falcate conidia each contained one nucleus, whereas the oval type contained one to four nuclei. A difference in cell wall structure was suggested by greater sensitivity of oval conidia to cell-wall degrading enzymes. Oval conidia were the only type produced in shake culture grown in the dark. Both types of conidia were produced by cultures on agar media and by standing cultures in small volumes of liquid medium but, unlike oval conidia, development of falcate conidia was light-dependent. When used to inoculate susceptible corn seedlings, both types of conidia caused symptoms comparable over a range of inoculum concentrations. Regardless of the type of conidia used to inoculate seedlings, both were found in lesions of infected leaves.