AbstractThe genetic diversity of three geographic populations of Phytophthora sojae from China and the United States was determined using technique of inter-simple sequence repeat (ISSR). A total of 102 reproducible ISSR fragments were scored among 111 individuals, of which 88 (86%) were polymorphic. Analysis of genetic variation showed that there existed higher genetic variation in the United States population in comparison to the Chinese populations. Nei’s genetic identity and principal component analysis indicated that the populations of Fujian and the United States are closer to each other than to Heilongjiang populations. Shannon-Wiener diversity index revealed that the United States populations have a higher genetic diversity than that of the Chinese populations. These data support the hypothesis that P. sojae in the United States might not have been introduced from China.