The goal of this study was to determine the expression profile of a late blight resistance gene, Rpi-phu1, after challenge with an incompatible Phytophthora infestans isolate. Relative expression levels were measured by quantitative real-time PCR, before as well as 1, 3 and 5 days after contact with the pathogen. Plants tested included diploid and tetraploid breeding lines with the gene in different genetic backgrounds and plants of a chosen breeding line at different developmental stages. All tested potato breeding lines with the Rpi-phu1 gene were highly resistant to P. infestans and so was the line tested at different developmental stages. No significant changes were detected in Rpi-phu1 transcript level after pathogen challenge in any of the tested tetraploid potato lines. However, Rpi-phu1 transcription in diploid lines was enhanced by contact with P. infestans. Basal expression of the Rpi-phu1 gene was significantly lower in the youngest, 3-week-old plants than in 6-week-old plants. Nevertheless, 1 day after inoculation with the pathogen, transcript levels in these plants were slightly greater than in 6-week-old plants, indicating that plant age can influence transcription of the Rpi-phu1 gene, but that this does not affect late blight resistance.