In this study defense responses in three potato varieties with different levels of reaction to the late blight disease caused by Phytophthora infestans were analyzed after inoculation with the pathogen.In the resistant cv.Pastusa Suprema, increased intensity of H 2 O 2 and callose deposit accumulation was observed beginning at 24 hours after inoculation, followed by a hypersensitive response at the inoculation points.In the moderately resistant cv.Diacol-Monserrate, the same responses were observed as in the resistant variety, but with less intensity over time.For the susceptible cv.Diacol-Capiro, the responses observed occurred later than in the other two varieties, subsequent to the advance of the pathogen over extensive necrotic areas.These results suggest that early, intense peroxide and callose accumulation and a hypersensitive response are associated with the observed resistance of the cv.Pastusa Suprema and cv.Diacol-Monserrate to P. infestans.