A collection of isolates of Phytophthora infestans (Mont.) de Bary was obtained from potato and tomato samples from across Canada in each of 1994, 1995, and 1996. Characterization of these isolates according to mating type (A1 or A2), sensitivity to metalaxyl, growth in culture, and allozyme banding patterns, revealed eight distinct genotypes of the fungus. In 1994, the US-1 genotype (A1, metalaxyl-sensitive (MS)) was commonly recovered in Canada (outside British Columbia). By 1996, the US-1 genotype was no longer recovered from any samples, and outside British Columbia, the US-8 genotype (A2, metalaxyl-insensitive (MI)) was the dominant genotype in Canada. In British Columbia, the g11 genotype (A1, MI) became the dominant genotype recovered in 1995 and 1996. There was no correlation between recovery of MI strains and metalaxyl use. Allozyme banding patterns were strongly correlated to mating type, metalaxyl sensitivity, and cultural characters, revealing the clonal nature of populations. Most isolates of recently introduced genotypes were more aggressive (had greater fitness) on tuber tissue than isolates of the US-1 genotype. Variation also occurred within a genotype and an isolate of the US-8 genotype from New Brunswick was consistently less aggressive than other US-8 isolates. A total of 28 pathotypes were found among 80 isolates of P. infestans tested. There was a significant increase in the complexity of pathotypes from 1994 to 1996, which reflected the displacement of the US-1 genotype (mean of 2.2 host differentials infected) by the US-8 genotype (mean of 8.1 host differentials infected). Both mating types of the fungus were found in 7 fields and one sample was found to have oospores of P. infestans visibly present in plant tissues. Therefore, P. infestans can reproduce sexually in nature in Canada. These results demonstrate that populations of P. infestans in Canada changed dramatically from 1994 to 1996 and the original US-1 (A1, MS) genotype was rapidly displaced. Migration of new forms followed by genetic drift operating through founder effects and selection for fitter genotypes are postulated as the primary mechanisms responsible for the observed patterns of evolution.