Tomato plants pre-colonised by the arbuscular mycorrhizal fungus Glomus mosseae showed decreased root damage by the pathogen Phytophthora nicotianae var. parasitica. In analyses of the cellular bases of their bioprotective effect, a prerequisite for cytological investigations of tissue interactions between G. mosseae and P. nicotianae v. parasitica was to discriminate between the hyphae of the two fungi within root tissues. We report the use of antibodies as useful tools, in the absence of an appropriate stain for distinguishing hyphae of P. nicotianae v. parasitica from those of G. mosseae inside roots, and present observations on the colonisation patterns by the pathogenic fungus alone or during interactions in mycorrhizal roots. Infection intensity of the pathogen, estimated using an immunoenzyme labelling technique on whole root fragments, was lower in mycorrhizal roots. Immunogold labelling of P. nicotianae v. parasitica on cross-sections of infected tomato roots showed that inter or intracellular hyphae developed mainly in the cortex, and their presence induced necrosis of host cells, the wall and contents of which showed a strong autofluorescence in reaction to the pathogen. In dual fungal infections of tomato root systems, hyphae of the symbiont and the pathogen were in most cases in different root regions, but they could also be observed in the same root tissues. The number of P. nicotianae v. parasitica hyphae growing in the root cortex was greatly reduced in mycorrhizal root systems, and in mycorrhizal tissues infected by the pathogen, arbuscule-containing cells surrounded by intercellular P. nicotianae v. parasitica hyphae did not necrose and only a weak autofluorescence was associated with the host cells. Results are discussed in relation to possible processes involved in the phenomenon of bioprotection in arbuscular mycorrhizal plants.