In order to accelerate the introduction of new potato cultivars (Solanum tuberosum L.) with durable resistance to late blight (Phytophthora infestans Mont. De By), and within the framework of standard international field trials, in the summer of 1999 twenty five potato cultivars were planted in plots with and without fungicides in the Valley of Toluca, México. Plots without chemical protection against the disease showed final disease severities ranging from 5 to 15 % (‘Norteña’ and ‘Pampeana’-INTA, respectively) with no statistical differences in this parameter. There were differences among genotypes, though, when comparing the area under the disease progress curve. This provided evidence for the presence of the oomycete in spite of fungicide spraying. Tuber yields averaged 1200 g per plant for the highest yielding cultivar Stirling, and 100 g per plant for ‘Monserrate’; there were statistical differences among genotypes. In plots without fungicides the least disease severity (22 to 27 %) was observed in ‘Norteña’, ‘LBr-20’, ‘Cruza-148’, and ‘Idiafrit’. On the other hand, the highest incidences (90 to 100 %) occurred in ‘Pampeana’-INTA, ‘Teena’, and ‘Bionta’. ‘Stirling’, with 53 % final foliar infection, produced the highest yields (933 g per plant) in plots without fungicide, followed by ‘Norteña’ (22 % infection, 675 g per plant), and ‘Rosita’ (38 % infection, 418 g per plant). The rest of the materials yielded less than 350 g per plant, and no direct relationship was detected between degree of disease severity and tuber yield. That is, genotypes resistant to the disease did not necessarily yield the most.