ABSTRACT--The current circumscription of Elymus based on cytogenetic analyses includes all allopolyploid Triticeae species containing the St (Pseudoroegneria) genome.  In North American Elymus, the St genome is combined with H (from Hordeum) in an allotetraploid (StStHH) configuration.  The goal of this study is to determine whether molecular phylogenetic analyses support existing cytogenetic data with regard to the evolutionary origin of North American Elymus.  Analyses were performed using sequences from the nuclear starch synthase gene, and include multiple species of Elymus, Pseudoroegneria, and Hordeum, along with representatives of most of the other monogenomic genera in the Triticeae.  The results support the hypothesis that Pseudoroegneria and Hordeum are the genome donors to the North American Elymus tetraploids.  One species currently placed in Elymus (an octoploid, Elymus californicus) appears to be unrelated to the rest.  The close relationships among ElymusH-genome sequences, with one exception, are consistent with a single origin of the group.  The St-genome group consists of two well-defined clades, but support for the monophyly of the entire St group is weak.  There are shortcomings associated with the current dependence on genome pairing data for grouping and ranking in the Triticeae, but molecular phylogenetic data suggest that, in many cases, groups delimited by cytogenetic data do in fact correspond to evolutionary lineages.