The production of pathogenesis related PR-b₁ protein has been compared in tobacco roots infected by the pathogenic fungus Chalara elegans and during symbiotic interactions with the vesicular arbuscular endomycorrhizal fungus Glomus mosseae. PR-b₁ protein could be detected by Western blotting of soluble protein extracts from pathogen-infected roots but not of endomycorrhizal tobacco roots. However, Northern analysis of total RNA extracts from endomycorrhizal roots revealed a single hybridization signal which was extremely weak when compared to that of C. elegans-infected tobacco roots. Indirect immunogold labelling showed that PR-b₁ antigen was very localized in endomycorrhizal roots and limited to host material in the interfacial matrix surrounding living intracellular fungal haustoria (arbuscules) in parenchyma cortical cells. Results are discussed in relation to PR-b₁ protein synthesis in pathogen interactions and host control in the endomycorrhizal symbiosis.