Phytophthora infestans causes late blight on both potato and tomato. In recent years, monitoring this pathogen became imperative because of its increasing aggressiveness and resistance to fungicides. Concurrent to these changes, dramatic shifts in the structure of P. infestans populations, including their genotypes, were recorded. To better understand such transitions and their potential impacts on potato, changes over time in the pathogenicity of this oomycete were investigated. Over 1200 isolates were collected during three growing seasons, from 1997 to 1999, and 87 of them were chosen based on their host of origin (potato or tomato), glucose phosphate isomerase genotype (Gpi 100: 111: 122 or Gpi 100: 100: 111), and geographic origin (Canadian regions). The selected isolates were tested on a series of 10 differential potato host lines, which carried single resistance genes, to determine their physiologic races and their relationship to the host, the region of origin, and the Gpi genotype. This study revealed the presence of 46 pathotypes of P. infestans among the 87 tested isolates. A higher variability was observed within the Gpi 100:111:122 genotype (33 races among 55 isolates), as compared with the Gpi 100:100:111 genotype (16 races among 32 isolates). Comparatively to previous years, the number of race combinations found in this study was relatively high, and many differences were found between Gpi 100: 1 1 1: 122 and Gpi 100: 100: 111 populations. In contrast, no differences were found between potato and tomato isolates in terms of their physiologic races.