Tomato cell clones selected for high and low (F+ and F−) response to heat-released cell wall components from Fusarium oxysporum f. sp. lycopersici were analysed ultrastructurally with particular regard to cell wall and membrane modifications in a synchronous dual culture in vitro system which avoided direct contact between the two partners of the interaction. Constitutive differences in cell shape were observed between F+, F− and parental cultivar cells by TEM and SEM. Quantitative morphological analyses, and biochemical determinations showed differences in cell wall thickness and polysaccharide content between cells of the F+ and F− clones. A temporal study revealed differences between the cell clones in (1) the rate of resistance to degradation by fungal polysaccharide hydrolytic enzymes, (2) protoplast destruction, (3) formation of paramural bodies, (4) cell wall appositions (papillae), and (5) mitochondrial alterations. The data are discussed with respect to a possible correlation between cell wall structure and the course of events during the host-parasite interaction on the one hand and, on the other, active responses to the pathogen as suggested by the literature.