A cytological study of Benzoin aestivale revealed the presence of several deviations from the usual meiotic process during microsporogenesis. These irregularities were considered to be diagnostic of previous hybridization of the species. The principal abnormality involved one of the twelve haploid chromosomes which appeared to be heteromorphic in both size and shape. In approximately half of the PMCs the larger of the heterochromosome elements was extruded from the heterotypic spindle and allowed to degenerate in the cytoplasm. As a result of the unequal distribution of the elements of the heterochromosome approximately two thirds of the functional pollen grains were calculated to carry the smaller element of the hetero-chromosome complex. The absence of a satisfactory degree of regularity seemed to exclude this case as being another X-Y sex chromosome mechanism. The meiotic behavior of the heterochromosome did suggest that it may represent an intermediate stage in the evolution of what are conventionally called sex chromosomes.