A hypertriploid (2n = 22) was detected in the progeny of a desynaptic diploid (2n = 14) plant of perennial ryegrass,Lolium perenne L. The hypertriploid did not differ in morphology from its maternal-sib diploids, but showed larger stomata and pollen. The microsporocytes showed a mean chromosome association of 4.3 I + 3.4 II + 3.2 III + 0.3 IV at metaphase I with a mean chiasma number of 14.4 per cell. The 33 types of observed chromosome configurations could be explained by assuming that the plant was trisomie for 6 chromosomes and tetrasomic for one chromosome. Karyotype analysis confirmed the above assumption and revealed tetrasomy of chromosome number VI. However, two chromosome associations, 1I + 4 II + 3 III + IIV and 2 I + 5 II + 2 III + 1 IV suggested the presence of displaced duplications within the genome of ryegrass. The tetrasomic chromosome formed mostly a quadrivalent, which often broke down to form 2 II or 1 I + 1 III and rarely 2 I + 1 II or 4 univalents. Most of the univalents arose from the trisomic chromosomes and divided precociously at anaphase 1, producing diads and tetrads with unequal chromosome numbers. The plant was highly sterile and set no seed on controlled crossing.