The phyletic relationship and the genetic isolating mechanisms between the Indian hexaploid S. nigrum L. and S. sarachoides Sendt. were determined by the study of meiosis in their F1 hybrids. The hybrids were highly sterile with irregular meiosis and did not set fruit. From cytological study of the hybrids it is suspected that polyploidy, structural hybridity and genic differences are responsible for reproductive isolation and morphological distinctiveness of the 2 spp. On the basis of occurrence of as few as 11 bivalents at metaphase I in some of the cells of the hybrids, S. sarachoides, or one of its close diploid relatives, might have contributed a set of 12 chromosomes in origin and evolution of hexaploid species of the S. nigrum complex. two species.