The subgenus Ceratotropis of the genus Vigna which originated in Asia contains cultigens such as mungbean, adzuki bean, etc. The Ceratotropis species are diploid (2n=22, 2x) except for the occurrence of two tetraploid species, V.glabrescens and V.reflexo-pilosa. V.glabrescens exhibits resistance to major mungbean pests and diseases including powdery mildew, cucumber mosaic virus, bean fly (Ophiomyia phaseoli, O.centrosematis and Melanagromyza soja), etc., and is useful in mungbean improvement programs. In order to clarify the phylogenetic differentiation of the tetraploid species, we analysed the isozyme banding patterns of the Ceratotropis species. Isozyme analysis suggested that wild V. reflexo-pilosa originated from interspecific hybridization between V.trinervia and V.minima var. minima followed by spontaneous chromosome doubling. Isozyme analysis and interspecific hybridization experiments showed that V.glabrescens and V.reflexo-pilosa are phylogenetically closely related. V.glabrescens is thus considered to be derived from V. reflexo-pilosa as cultivated type with erect growth habit. The data obtained from interspecific hybridization experiments showed that V.trinervia was the seed parent rather than the pollen parent. V.reflexo-pilosa, V.minima var. minima and V.trinervia are also anticipated to be useful for breeding programs of Ceratotropis cultigens in addition to V.glabrescens, since they are involved in the speciation of V.glabrescens.