A detailed comparative cytomorphic study of the diploid Ipomoea biloba Forsk. (2n=30) and a naturally-occurring tetraploid form (2n=60) was made. The extent of morphological differences between the two chromosomal types have been presented and discussed. These differences relate to leaf shape and size, pollen size and length of seed coat hair. The course of microsporogenesis was investigated from pachytene stage onwards in both the taxa. Based on these data and their geographical distribution, the mode of origin of the tetraploid taxon and its relationship to the diploid was considered. The close similarity in their exomorphic characters, presence of both the diploid nucleolar chromosomal types in tetraploid and the sympatric distribution of the two taxa are advanced as evidences in favour of the direct role played by diploid biloba in the origin of the tetraploid taxon. However, in view of the prevalence of preferential pairing, low multivalent frequency and high fertility, a segmental allopolyploid origin seems probable.