Because of the greatly increased supply of genetic variation which it provides, introgression is of great importance in determining both the rate and the direction of evolution. Hybridization between the genera Cowania and Purshia provides an excellent model for demonstrating the various products which might be expected from introgression. The present study is an attempt to further investigate phylogenetic relationships of Cowania mexicana and Purshia tridentata by use of cytological and biochemical techniques.