Cytoganetical investigations were carried out in some species of Ocimum. During the course of this investigation it was found that O. canum Sims and O. americanum Linn., which have long been considered as synonym, are two distinct species. The former has 2n=24 or its aneuploid form with 2n=26 chromosomes while the latter has 2n=72 chromosomes. It is suggested that O. americanum arose as a natural hybrid between the diploid O. canum (2n=24) and the tetraploid O. basilicum (2n=48) followed by doubling of chromosomes. An experimental evidence to support this hypothesis is provided by resynthesizing O. americanum from O. canum and O. basilicum. The interrelationship and evolutionary aspects of the species are also discussed.