Samples of 4285 individuals from a hybridSolanum phureja Juz. et Buk. —S. stenotomum Juz. (Phu-Stn) and 105 individuals ofSolanum phureja (Phu) diploid potato populations were twice inoculated with potato virus Y strain “o” (PVY∘) using the air brush technique. After discarding seedlings with PVY visual symptoms both in the greenhouse and in the field, 1508 seedlings were judged to be resistant to PVY∘ (35.2%). Only 5.7% of the Phu seedlings were PVY∘ resistant compared to 35.2% of the Phu-Stn seedlings. At harvest, a mild selection pressure for yield and tuber appearance was applied and 602 Phu-Stn clones were chosen for further evaluation. Selected clones were re-evaluated for PVY∘ resistance in the greenhouse. Clones were mechanically inoculated twice with PVY∘. Clones susceptible to PVY∘ were determined by visual observations, ELISA (Enzyme-linked immunosorbent assay) tests, grafting of tobacco PVY∘ infected shoots on potato clones, and infectivity tests usingNicotiana tabacum as a PVY∘ plant indicator. In the process, 224 clones were found to be extreme resistant to infection by PVY∘, with an overall frequency for PVY∘ extreme resistance of 5.2%. In another experiment, the first year Phu-Stn PVY∘ resistant clones (554 clones) were mechanically inoculated twice with “common” strain of potato virus X (PVX). Similarly, we discarded clones susceptible to PVX by combining visual evaluation and ELISA with PVX re-inoculation of negative clones and an infectivity test, usingGomphrena globosa as a PVX plant indicator. After this process, seven extreme resistant and eight resistant clones to infection by PVX were found; the overall frequency of PVX extreme resistance was 1.3%. Five clones were extreme resistant to both PVY∘ and PVX.