Phytophthora ramorum (Oomycetes) is an emerging plant pathogen in forests in southwestern Oregon (Curry County). Moreover, since 2003 it has been repeatedly isolated from plants in Oregon nurseries. In this study, we analysed the genetic diversity of the P. ramorum population in Oregon from 2001 to 2004 by using microsatellites. A total of 323 isolates (272 from the infested forest; 51 from nurseries) were screened at 10 loci. The overall P. ramorum population in Oregon is characterized by low genetic diversity and has all the hallmarks of an introduced organism. All isolates within the A2 mating type belonged to the same clonal lineage and no recombinant genotypes were found. The forest population (24 genotypes) was dominated by a single multilocus genotype which persisted over years, indicating that eradication efforts in the forest have not completely eliminated inoculum sources. In contrast, genotypic evidence suggests that eradication was effective in nurseries. In 2003 and 2004, a total of 11 genotypes were found in the nurseries (one belonged to the European lineage of P. ramorum) but no genotype was recovered in both sampling years. Significant differentiation and low gene flow were detected between nursery and forest populations. Only two nursery genotypes were also found in the forest, and then at low frequency. Thus, the nursery infestation is not caused by the genotypes observed in Curry County, but likely resulted through introduction of novel genotypes from nurseries out-of-state. This highlights the continued importance of sanitation and quarantine in nurseries to prevent further introduction and spread of P. ramorum.