We determined the parental species of Youngia koidzumiana (a natural interspecific hybrid) using PCR and arbitrary 10-mer primers to generate random amplified polymorphic DNA (RAPD) markers. These markers, generated by three primers, were sufficient to distinguish Youngia sonchifolia, Younsia denticulata, Youngia chelidoniifolia, and Y. koidzumiana. The electrophoresis profiles of the amplified products from each of the four species were then compared. Three primers produced a total of 42 scorable markers; nine were specific markers for Y. denticulata and Y. chelidoniifolia. The length of the amplified DNA fragments ranged from 370 to 2500 b p. The three primers revealed polymorphic bands, which were indicators of the parental species of Y. koidzumiana. These bands showed a combination of specific profiles for Y. denticulata and Y. chelidoniifolia. Our results also were comparable to the data obtained for flowering times, floret numbers, and chromosome numbers of the four species. Therefore, we suggest that Y. koidzumiana is a hybrid between Y. denticulata and Y. chelidoniifolia, and that RAPD markers are well suited for assessing the origins of plant species.