The effects of combinations of post-inoculation temperature (15, 25 "C) and light intensity (100, 500, 1000 uEm-Y') on disease severity (three parameters), induced by six races of Melampsora medwae in four cultivars of Popuh were assessed in a factorial experiment. While all cultivars were immune when incubated at 25 "C and 1000 uEm-*s-i, most were susceptible, to varying degrees at all other temperature/light intensity combinations. Although the relative rating for disease severity of cultivar/race. combinations depended to a degree on the parameter used for assessment, the temperature of incubation, irrespective of the parameter, was the most important determinant of variation in severity. All the major variables, and their twoand three-way interactions, were significant determinants of variation in number of uredinia developed per unit leaf area. The ranking of cultivars for relative resistance, and of races for relative aggressiveness, depended on the temperature/light intensity treatment during incubation. The lability to environment, of the cultivar/race reactions in this pathosystem, could contribute to stability in resistance to leaf rust in plantations of poplar.