The synthetic amphiploid cv Prior was created in the early 1970s at the Welsh Plant Breeding Station by crossing colchicine-induced autotetraploids of Lolium perenne (2n=14) and Festuca pratensis (2n=14). Meiosis in the early generations was characterized as stable, with frequent bivalent formation. In situ hybridization of a L. perenne total genomic DNA probe to mitotic chromosome spreads of 12 plants, from two extant populations of Prior, demonstrates extensive recombination between the two genomes. Recombination events occur along the whole length of chromosome arms but with a higher frequency in the medial portion. The species origins of chromosomes were assigned by the presence or absence of a fluorescent probe at the centromere. There has been a substitution of Festuca-origin chromosomes by those of Lolium-origin, resulting in a mean of 17.9 (15–21) Lolium and 9.7 (7–13) Festuca chromosomes per genotype. Mean chromatin length per genotype comprised 62.1% Lolium and 37.9% Festuca. On average 9.3 Lolium (51.1% of those present) and 3.5 Festuca (37.8%) chromosomes had no recombined segments. For chromosomes which did show recombination, fewer alien segments were observed in Lolium than in Festuca chromosomes. Festuca chromosomes in genotypes selected for drought resistance had undergone more recombination than in genotypes from an unselected population, though this difference was not statistically significant for the small sample examined.