Rhododendron calendulaceum (2n=52) and R. bakeri (2n=26) constitute a semicryptic species pair of red to yellow-flowered deciduous azaleas found in the Appalachian Mountains. Two possible phylogenetic relationships have been suggested. either R. bakeri is one of the putative parents of teh allotetraploid R. calendulaceum, or R. bakeri is derived from R. calendulaceum through chromosome elimination. Three mechanisms describing how diploids can be derived from tetraploids are set forth: polyhaploidy, mitotic chromosome elimination, and complement fractionation. The possibility of polyhaploidy is discounted, while new evidence is presented to support the existence of complement franctionation in R. calendulaceum hybrids.