Evidence, based on ultrastructural observations of stages involved in root infection oi Nicotiana tabacum cv. Xanthi n.c. in vitro by the black root rot fungus Chalara elegans, indicates that host cells from various layers react differently when challenged by the pathogenic fungus. All the host responses observed were associated with host cell wall modifications. Host reaction to fungal invasion occurring in the epidermal cells was limited to a disorganization of the cytoplasm. In the hypodermal cell layer, fibrillar cell wall outgrowths and wall thickenings were the earliest and the most obvious host reactions. In parenchymal cells, the host reacted by depositing papilla‐like wall appositions directly adjacent to the infecting hyphae; with secondary infection of these cells, a densely staining material was laid down, mainly around the distal region of the infecting hyphae. In all these tissues, infection also led to disorganization of the host cytoplasm. Colonization of the endodermis did not lead to any rapid lethal modifications in either the host or the fungus, and a biotrophic‐like state seemed to occur at this stage of the infection. No hyphal infection occurred in the central cylinder.