Four strains of PVX from Peruvian potatoes were inoculated to 17 species of indicator plants and to several potato cultivars and wild tuber-bearing Solanum species. Only one induced the typical symptoms of this virus in indicators. The rest induced mild mosaic in most solanaceous hosts. Dilution end-point, thermal inactivation point and longevity 'in vitro' were, in general, within the expected range for this virus; however, one strain had a dilution end-point above 10-6 and another, a thermal inactivation point below 66°C. All Peruvian cultivars tested were susceptible and 4 sometimes reacted with top necrosis. It was possible, from this top necrotic reaction, to deduce that the 4 strains belong to groups 2 and 3 of Cockerham's classification of PVX strains. The highly resistant clone USDA 41956 was also resistant to all these strains. © 1975 Kluwer Academic Publishers.