Lolium perenne (4x)/Festuca pratensis (2x) triploid hybrids (2n = 3x = 21) were produced and backcrossed to Lolium perenne. The BC1 progeny, which predominantly had 14 chromosomes, were analysed using genomic in situ hybridization (GISH) and genetic markers. GISH revealed that over 74% of the BC1 individuals carried one or more F. pratensis chromosome segments. By comparing the physical size of introgressed F. pratensis chromosome segments with the presence or absence of F. pratensis-specific polymorphisms, it was possible to determine the physical position of genetic markers. The potential of a new type of genetic mapping ('introgression mapping') for the alignment of physical and genetic maps, determining the genetic control of agronomically important characters and the production of novel germplasm for the development of new varieties is discussed.