Gene flow into populations of Phytophthora infestans in the United States and Canada was detected directly in 1992 and 1993. In total, 384 isolates were analyzed for mating type and dilocus allozyme genotype. Of these, 130 isolates were analyzed for nuclear DNA fingerprint variation using probe RG57. Only 11 multilocus genotypes were identified, nine of which had never been detected previously in the United States or Canada. The A2 mating type and isolates with the Glucose-6-phosphate isomerase 111 allele increased dramatically in frequency compared with samples in previous years. The only likely explanation for these changes is that there was massive immigration from outside the United States or Canada. Northwestern Mexico was the probable source population for these recent migrations; the two most common migrant genotypes were probably direct clonal descendants of isolates collected previously in northwestern Mexico. Populations in 118 of the 122 sites sampled appeared to be monomorphic. Thus, the genetic structure of epidemic populations in most sites was very simple and probably exclusively asexual. The two mating types were usually separated geographically. However, both mating types were detected together in three fields in British Columbia. Some isolates from these fields had unique genotypes that could have arisen by sexual recombination between the most common A1 and A2 genotypes in British Columbia. This is the first evidence for the probable occurrence of sexual reproduction of P. infestans in North America north of Mexico