Conidiogenesis in Septoria chrysanthemella was studied in vitro with light and transmission electron microscopy (TEM). For the first time, proof is given with TEM that both percurrent and sympodial proliferation can occur in a single conidiogenous cell. Ontogeny of conidia is holoblastic. After delimitation by a transverse, uniperforate septum, each conidium is liberated schizolytically. Three kinds of conidiogenous cells occur in S. chrysanthemella: (i) annellides, proliferating only percurrently; (ii) sympodulae, proliferating only sympodially, and (iii) cells proliferating either way. No influence of illumination by diffuse daylight or nuv, or medium was observed on qualitative aspects of conidiogenesis. The annellations in S. chrysanthemella seen with TEM are not resolved by light microscopy (LM). It is concluded that LM is not sufficient to assess conidiogenesis in species of Septoria with equally minute conidiogenous cells.