Cytological and physiological studies were conducted on the infection process of Colletotrichum sublineolum P. Henn. Kabat et Bub. on susceptible and resistant cultivars of Sorghum bicolor (L.) Moench. Compatible interactions were characterized by an early biotrophic phase lasting c. 24 h, during which intracellular infection vesicles and primary hyphae colonized epidermal cells. Thinner secondary hyphae were first observed branching out from primary hyphae after 66 h. These hyphae proliferated throughout the epidermis and into underlying mesophyll cells. Secondary hyphae were associated with host cell death and the onset of a necrotrophic phase. Pigmented cytoplasmic inclusions, thought to represent the host defence response, were also first observed at 66 h, although there was little apparent effect on subsequent fungal colonization of tissue. By 90 h, secondary hyphae had proliferated throughout the leaf sheath, with production of necrotic lesions and acervuli. In incompatible interactions, infection vesicles formed inside epidermal cells within 42 h of inoculation but there was no evidence for a biotrophic interaction as penetrated cells appeared dead and fungal development was restricted to single epidermal cells. Host defence responses were observed earlier than in compatible interactions: heavily pigmented (dark red) cytoplasmic inclusions were distributed throughout penetrated cells at 42 h. By 66 h, these cytoplasmic inclusions had disintegrated and dark red pigment had spread throughout the cell. Concurrent with the disintegration of the cytoplasmic inclusions was the disruption of the cytoplasm of both the host cell and the fungus. Further fungal development was restricted. These observations are discussed in relation to the importance of the initial biotrophic phase in the successful colonization of S. bicolor by C. sublineolum and the role of the host defence response in preventing colonization.