Subtelomeric regions of chromosomes are particularly dynamic and variable parts in the evolution of the eukaryotic genomes. A specific feature of the rye (Secale) chromosomes is large heterochromatin blocks at the arms of all seven pairs of chromosomes. Within the genus Secale, an interspecific variation in the genome size reaches nearly 15% between Secale cereale (cultivated rye) and the ancient species S. silvestre and an increase in the size of subtelomeric heterochromatin regions is the main contributor to these differences. Earlier works have demonstrated that the subtelomeric hetechromatin of rye is enriched for a few multicopy tandemly organized DNA families which form the long arrays of monomers. Here we aimed to clarify the fine large-scale organization and mutual arrangement within the tandem arrays of these families and the flanking genomic nonarray DNA. Restriction analysis of the BAC-clones containing genetic material of the short arm of the first rye chromosome (1RS) showed that within arrays the bulk of monomers is organized in the specific higher-order repeat units (up to hexamers) which are generated in the central part of tandem arrays, while only monomers and dimers are present near the boundaries. Sequencing of the genomic nonarray DNA flanking the tandem arrays has demonstrated that this DNA in the studied BAC-clones consists completely of retrotransposons of various classes which are also present in the wheat and barley genomes. Thus, only an explosive amplificcation ofpSc200 and pSc250 monomers, on the background of a saturated mixture of various retrotransposons, can be regarded as a specific molecular process in formation of subtelomeric hetechromatin during the divergence of rye genome from the common ancestor of cereals. Evidently, this process resulted in the enlargement of subtelomeric hetechromatin of S. cereale and an increasing of its genome size.