Diploid plums (Prunus L. sp.) and their progenitor species were characterized for randomly amplified polymorphic DNA polymorphisms. Bootstrap analysis indicated the variance of genetic similarities differed little when the sample size was >80 markers. Two species from China (Prunus salicina Lindl. and P. simonii Carr.) and one species from Europe (P. cerasifera Ehrh.) contributed the bulk (72% to 90%) of the genetic background to the cultivated diploid plum. The southeastern plum gene pool was more diverse than those from California, Florida, or South Africa because of the greater contribution of P. cerasifera and P. angustifolia Marsh. to its genetic background.