The genetic diversity of three geographic populations of Phytophthora sojae from China and the United States was determined using random amplified polymorphic DNA (RAPD). The purpose was to explore genetic relationships among Chinese and American isolates of the organism. 21 random primers were selected among 200 random primers screened. A total of 223 reproducible RAPD fragments were scored among 111 individuals, of which 199 (89.23%) 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 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 Chinese populations. These data are in support of the hypothesis that P. Sojae in the United States might not have been introduced from China.