The phytogeny of Festuca arundinacea Schreb. (2n = 6x = 42) was determined using GISH. Total genomic DNA of putative ancestral species was labelled with rhodamine and hybridized to chromosome preparations of hybrids involving these species and F. arundinacea. The degree of hybridization to chromosomes known to be homologous to the probe DNA was compared with that found simultaneously on chromosomes of the genome of F. arundinacea. It was concluded that the tetraploid species Festuca arundinacea var. glaucescens contributed two genomes and the diploid species Festuca pratensis one, to create the allohexaploid species F. arundinacea.