Abstract: The sensitivity of 15 lines of tomato (Lycopersicon esculentum) to six isolates of the late blight fungus (Phytophthota infestans) was assessed by a detached leaflet method at several plant ages. Most lines showed variation in reaction at different ages; FMX-93 and LA 1338 were consistently sensitive to all isolates whereas LA 1623 was hypersensitive to all isolates except Ca65 which attacked these and all other lines. Isolate 88/15 showed consistent virulence to most tomato lines and avirulence to LA 1623 and Rockingham. Accessions Rockingham and LA 1623 were crossed to susceptible accessions. When screened with isolate 88/15, all F1 plants of both crosses were resistant. F2 families of each cross segregated to produce approximately 25 percent susceptible plants. A single dominant gene conferring resistance is thus indicated in each resistant parent. This was confirmed in backcross families which segregated 1:1 for resistance: susceptibility.