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Two P. infestans populations in northern Mexico (northwestern and northeastern) were analysed for genetic variation by use of mating type, 2 allozyme loci, and 2 DNA fingerprinting probes, and were compared with 2 populations from central Mexico. The genetic structure of the populations varied widely, even though both mating types occurred in all locations sampled. There was very little genetic diversity in northwestern Mexico; 4 genotypes were detected among 88 isolates tested, and only 2 of these were common. All A2 isolates in this population appeared to represent a single clone and were exclusively on potatoes, whereas the A1 isolates were primarily on tomatoes. As the A2 mating type was not known previously in northwestern Mexico, this clone probably represents a recent expansion in its range. In contrast to the low diversity in northwestern Mexico, isolates from northeastern and central Mexico …