Cellular events that occur during the initial interactions between Thielaviopsis basicola and root hairs of tobacco (Nicotiana tabacum) were examined microscopically. Time-course documentation of the infection process indicated a dynamic interaction between T. basicola and the living host cell. Upon root hair contact and recognition, the vegetative apex of T. basicola rapidly differentiated to form infection structures, and the host cell responded cytologically. Penetration was achieved by threadlike hyphae that subsequently developed distal swellings, and intracellular hyphae of sickle-shaped morphology advanced from the distal swelling and colonized the cell. Streaming of the host cytoplasm became aggregated near the infection site prior to penetration and accumulated around the infecting hyphae as long as the host cell was viable. Substantial callose deposition, in the form of a bell-shaped collar around infection structures, resulted from the cytological activity at the infection site. Penetration of dead root hairs was common, but did not lead to the development of infection structures or to a sustained association with the host tissue; T. basicola exited dead root hairs and resumed vegetative growth. The establishment of the parasitic relationship by T. basicola was characteristic of hemibiotrophic fungi in that, initially, biotrophic infection led to tissue colonization, and host cell survival was limited under parasitism.