The expression levels of PR‐b1′ gene coding for PR‐b1′ protein (a PR‐1 group protein) were investigated in the roots of two Nicotiana species and their amphidiploid hybrid in response to infection by virulent (84–1) or avirulent (85–26) races of black root rot fungus Chalara elegans. Nicotiana glutinosa is susceptible to C. elegans, while Nicotiana debneyi and the amphidiploid hybrid N. glutinosa×N. debneyi are highly resistant. The results showed that irrespective of whether the plant is susceptible or resistant, and the fungal race virulent or avirulent, low infection rates induce low levels of PR‐b1′ mRNA, and high infection rates result in high production of PR‐b1′ mRNA. The amphidiploid hybrid, which expresses the protein PR‐b1′, constitutively, showed similar PR‐b1′ mRNA levels with both races of C. elegans. At maximal levels of accumulation, PR‐b1′ mRNA is quantitatively similar in the two Nicotiana species, and the amphidiploid hybrid, whatever the nature of the inoculum.