We have used total genomic DNA as a probe to size-fractionated restriction enzyme digests of genomic DNA from a range of Triticeae species from the genera LeymusHochst., PsathyrostachysNevski, and Hordeum L., and hybrids between Hordeum and Leymus to investigate their taxonomic relationships. Genomic Southern hybridization was found to be an effective and simple way to assess the distribution and diversity of essentially species-specific and common, repetitive DNA sequences, and is hence especially useful in evolutionary studies. The DNA sequences of H. vulgare seem to diverge substantially from those of H. brachyantherum, H. lechleri, H. procerum, and H. depressum. The genome of Thinopyron bessarabicum shows little homology to those of the Leymus species investigated, confirming that T. bessarabicum is not an ancestral genome in Leymus. Although the genomes of Leymus and Psathyrostachys share substantial proportions of DNA sequences, they include divergent repeated sequences as well. Hybridization with a ribosomal DNA probe (pTa 71) showed that the coding regions containing structural genes encoding the 18 S, 5.8 S, and 26 S ribosomal RNA were conserved among the species investigated, whereas the intergenic spacer region was more variable, presenting different sizes of restriction fragments and enabling a classification of the species. The rye heterochromatin probe pSc 119.2 hybridized to DNA from H. lechleri and T. bessarabicum, but not to DNA from the other species investigated.