In common with many other eukaryotic species, the genomes of species in the genus Allium contain a high proportion of repeated DNA sequences, which may be implicated in the considerable differences in genome size that are seen between even very closely related species. The gross organization of repetitive sequences within the genome of Allium sativum and of some other related species has been investigated using DNA/DNA hybridization studies. Such studies show that there has been much modulation in the amounts of different repeated DNA families during the evolution of the genus Allium; these repetitive elements are interspersed in all species with sequences of low repetition. The organization and distribution of one particular repetitive family within the genus has been examined using a cloned hybridization probe. Hybridization of this probe to DNA from related genomes reveals that this element is present in all Allium species examined, but with large-scale modulation of its abundance, and some considerable changes in its sequence environment. The evolution of such genome-specific arrangements of common repetitive elements and the possible mechanisms by which they might be maintained are discussed.