Hybridization frequency was investigated between tetraploid perennial and Italian ryegrass (Lolium perenne X multiflorum) without emasculation by using genetic markers. The Italian phenotypes, fluorescentroots and awned florets, were dominant. About 82% of the plants in perennial X Italian and nearly 93 % of the plants in the reciprocal crosses were hybrids. The hybrids had a high multivalent frequency and involved homoeologous chromosome pairing. Aneuploids with 2n = 26, 27 and 29 chromosomes were present. The hybrids were highly fertile. The cytogenetic behaviour of these allopolyploids suggested that the genomes of the parental species have undergone little repatterning and have free genetic exchanges. The species maintained their self-incompatibility and cross-compatibility at the tetraploid level.