The ability to detect small low- or single-copy DNA sequences by fluorescence in-situ hybridization (FISH) is an important step towards physical mapping of plant genomes. In this study, the FISH technique was used to physically map the Glu-1 loci controlling high-molecular weight (HMW) glutenin in common wheat (Triticum aestivum cv. 'Chinese Spring') and tritordeum (an amphiploid between T. turgidum cv. durum and Hordeum chilense). The probe used was the single-copy Glu-D1-1d gene coding the 1Dx5 HMW glutenin subunit. Three loci were mapped on chromosomes of wheat homoeologous group 1 (arm 1AL, 1BL and 1DL). The Glu-1 loci were mapped (fraction of the distance from the centromere) at positions 0.76 +/- 0.01, 0.69 +/- 0.01 and 0.76 +/- 0.01, on arms 1AL, 1BL and 1DL, respectively. The Glu-1 loci were also mapped on chromosomes of homoeologous group 1 of tritordeum at positions 0.75 +/- 0.01, 0.70 +/- 0.01 and 0.60 +/- 0.01, on arms 1AL, 1BL and 1HchL, respectively. Chromosomes with positive signals were identified by reprobing chromosome preparations using both the GAA-satellite and pAs1 sequences simultaneously. The application of the FISH technique to study homoeology among different genomes is discussed.