The influence exerted by the non-pathogenic binucleate Rhizoctonia (np-BNR) isolate 232-CG in stimulating plant defence reactions in young bean plants inoculated with the root rot fungus Rhizoctonia solani (AG-4) was examined using light and electron microscopy and further investigated by gold cytochemistry. Severe necrotic lesions on hypocotyles of diseased beans were observed, and the pathogen invaded the cortical tissue causing extensive damage including cell disorganization and cell wall degradation. In contrast, these host reactions were not seen in bean plants inoculated with the non-pathogenic BNR or in plants that were inoculated with BNR and subsequently challenge-inoculated with R. solani. Microscopic examination of hypocotyls inoculated with the non-pathogenic BNR, showed a different host reaction typical of plant defence reactions. In these samples, epidermal and outer cortical cells were impregnated with an electron-dense material. Histochemical assays of this material confirmed the substantial presence of phenols, pectic substances and suberin. Electron microscope observations clearly showed that in non-pathogenic BNR-inoculated plants, fungal cells were confined to the epidermal layer which was darkly stained. Gold cytochemistry confirmed the presence of pectic substances in the electron dense material. The possibility that pectic oligogalacturonides released after hydrolysis by the non-pathogenic BNR enzymes may act as elicitors of defence responses is discussed. The present ultrastructural observations corroborate that non-pathogenic BNR isolates may function as potential inducers of plant disease resistance.