ABSTRACT--Under the genomic system of classification, Elymus is defined as one of several allopolyploid combinations that include the St genome of Pseudoroegneria.  The present paper focuses on 1) native North American allotetraploid species of Elymus, all of which are presumed to combine the St genome with the H genome of Hordeum in an StStHH allotetraploid configuration, and 2) the introduced, weedy E. repens, an allohexaploid whose genomic constitution is not well understood, but which may include the St and H genomes.  The genomic complements of these species are examined in a phylogenetic context, using three data sets from the chloroplast genome and one from a nuclear gene encoding granule-bound starch synthase.  The data from the native Elymus species are consistent with cytogenetic data, and suggest that Hordeum and Pseudoroegneria are involved in the origin of the native North American allotetraploid species of Elymus.  The more preliminary results from Elymus repens are complex, and indicate  the involvement of  at least four genome donors, including Taeniatherum and an unknown donor in addition to Hordeum and Pseudoroegneria.