Genome relationships between the genera Leymus Hochst., Psathyrostachys Nevski and Hordeum L. (Poaceae, Triticeae) were investigated by fluorescent in situ hybridization using both total genomic DNA and cloned DNA sequences as probes. In hybrids between species of Hordeum and Leymus there was a clear differentiation between the H genomes of Hordeum species and the genomes of Leymus species after probing with genomic Hordeum or Leymus DNA. Chromosomes of species of Leymus and Psathyrostachys were also differentiated by subtelomeric heterochromatic segments or by negative bands along their length. The number and location of 18S-5-8S-26S rRNA genes varied between the investigated genera. Unusually, L. angustus and P. stoloniformis rDNA sites were localized on both ends of some chromosomes. Interphase nuclei of the Hordeum x Leymus hybrids had groups of chromosomes from both parental genomes in discrete, non-intermixed domains.